The following is all my opinion:
Sorry this will be long. Yet to the point. In my opinion, I find it pretty poor tactical thinking on anyone's part who would consider giving this thug aid under the scenario that was explained. I believe that aid should not even cross your mind at the point where it was left off for you to decide your next actions.
You have an armed intruder in your home; he has been shot or so you think. You are covering him with firearm at the ready. He has been shot in the thigh - and supposedly you can see this! Not a big chance of that, maybe you watch too many movies but my guess is that it is night, sort of dark in your abode and for all you know the guy just twisted his leg and fell or, maybe he cut himself with that knife. Are you sure he was shot, are you sure he is even bleeding, maybe you shot his accomplice (the one you did not see who is lying outside your window dead) and the guy still inside on your floor is faking because he got bloodied - think about that. Yet, I will play along and say ok you can tell you shot him in the thigh. You can also tell he is bleeding a lot. The thing is though that unless the blood is spurting from the wound with every heartbeat - you cannot tell that an artery was hit and cannot figure that a tourniquet is needed to save his life - so why even surmise that such is the case?
You have no way of telling if it is a life threatening wound or a grazing surface wound that is bleeding badly in that type of scenario unless maybe he is wearing shorts or is naked. Even then you do not know the extent of the damage. Add to that he is still holding the knife, he is obviously conscious. Why is he still holding that knife? He could be howling like a banshee too - so what. Maybe he is a cry baby, maybe he is diverting your attention. Most break ins, burglaries, armed robberies, strong arm jobs and so forth are carried out by more than one person. There is often another guy or two or even three involved. You may not see them, but they may be in your home and you are worrying about giving this guy first aid! He is still holding a knife, he has not been neutralized (I do not mean killed, but neutralized as far as a threat goes) and you are concerned as to whether he needs first aid; that is sweet of you but you had better make sure you are ok and safe first. Your wife is present, and possibly still in harm's way, as are you potentially still in harm's way.
First off, get distance and cover between you and the bad guy - both you and your wife. You want this between you and the bad guy in case he gets up and charges you, or in case he pulls out a Roscoe and starts firing. As you move to cover, order the bad guy to drop the knife, to slowly put his hands up high so you can see them and, to shut up and listen to you. Disregard his screams of pain and cries for assistance - it could be a deadly ruse, it could be real - that is not your concern while he is still armed and the house has not been cleared. Also make sure to scan the area or have your wife scan the area, look for other threats. Does the bad guy have friends with him. Does the bad guy have a firearm? Does he have anything else to hurt you? Don't just look, listen too.
Once you determine that he is at least less of a threat because he has dropped the knife, you keep him covered. By the way, how close to him is that knife? Have him toss it to the side away from him and you (why chance him throwing it at you).
Do not hand the gun to your wife. You are in control at this point and handing that away gun is a momentary opportunity for the bad guy to attack or to try to escape. In the confusion you or your wife could be shot as the gun goes from one to another. Instead, calmly ask your wife: "Are you injured?" Be rude to her and do not look at her while talking to her, keep your eyes on the bad guy and his hands with an occasional quick glance around the room. Do not allow yourself to get tunnel vision. Have her check herself for injuries and then check you - as you keep your eyes and firearm trained on the bad guy. Have your wife turn on some lights if possible in a safe manner. Then look around for threats again. As you do all this bear in mind even if you did not call the police a neighbor may have done so, they may be coming in soon, be aware of that.
Once you think you are fairly ok, and think the situation is fairly under control, take it one step further, give this order in a loud commanding voice: OK we know you are here too, we heard you moving. When I give the order, come out slowly with your hands raised high above your heads do it one at a time and remember we are armed. When ready give the command: "OK come out now one at a time slowly with hands held high. Move to the far side of the room from my wife and I and sit on the floor hands on your heads. We will not shoot if you do as we say." (Note you keep saying the "WE", this is to make them think you are both armed.) Then wait a minute or two to see what happens. If no one comes out, so what; if someone else is there and comes out hands up or, if someone runs out your back door to escape, won't you be happily surprised. If no none else was there - who cares.
If others came out, look for weapons from your vantage point, but do not approach them. Tell them you see the weapon, or the bulge and to keep their hands on their heads. Have them face away from you. They do not need to see that the wife is unarmed. You do not need to see their eyes at this point, only their hands. Once the situation is at this point and somewhat under control, have your wife make a call to the police if not already done - hopefully you already did this from the bedroom and, hopefully you told the police exactly what you and your wife looked like and you made sure they understood that was how the good guys were dressed!
Note how the husband has been the one giving commands. Only one person does that. Note how the wife has been carrying out orders, this keeps her busy and keeps her from panic as it also keeps you from panic. Remember it could be reversed with the wife holding the gun and giving orders, and the husband doing the other things - no real difference there. Do not argue over who does what. Have a plan ahead of time, discuss this type of thing and make a plan and practice it at least a few times a year.
Chances are the cops are there by now. They are outside waiting to come in and are ordering things to be done. You do not want to be pointing a gun at them by mistake. Stay calm, talk to them even before they come in. Let them know who you are and how many, tell them that you are armed and have a wounded and potentially armed and potentially still dangerous suspect. Ask what to do and follow there instructions. Adrenaline will be pumping fast and furious. You will be nervous even if you do not realize it - so will the cops. When it is over, once the bad guy is really under control and in the back of the police car you should immediately ask for medical aid to be rendered to you and your wife in a hospital and, also ask for an attorney. Make no statements to the police except for personal history information such as name and date of birth. You are not being a bad guy, just a smart victim. Politely tell the officers that you will probably talk to them after speaking to an attorney, but right now you need to go to a hospital because you want medical attention due to your not feeling well - and believe me you will not be feeling all that well if this really happens to you - even if you are the winners. After any such shooting, you should also seek immediate psychological counseling. You may not think you need it but, the sooner you get it the less likely you are to suffer post traumatic stress related problems.
If that one bad guy was the only one there, and if he bled out while you checked and tried to assure for your safety because you were still in fear of serious bodily harm or death, well in my mind that is tough noogies for him. My guess is that it probably would stay that way in criminal court and even in a civil court too so long as you could truly explain that you took the actions you took to assure for your safety and that of your family and you reasonably feared for your safety at that time - in other words you were scared and did what you did to protect yourself in a bad situation and you waited for the police and medical teams to assist the bad guy. Note this: As this situation was explained, if you were not fearful of possible serious bodily harm or death at the hands of this lowlife thug while he still held that knife and; if you were not fearful or at least cautious enough to scan to look for other threats in your home; and if you did not take the time to secure yourselves from harm and then call for the police, well then I think you need to reconsider the whole scene because at any moment you, or your wife, could have been killed in a heartbeat because of your carelessness and bravado. Of course, maybe all was well at the point in the original scenario where you thought you had to make up your mind about aid, but the fact of the matter, as I see it, is that the point where such a decision would have to have been considered was not yet arrived at as described. You had lots of work to do before getting to that point.
Oh, did I forget to mention, if he had not dropped the knife when ordered to do so, and if he seemed a real and immediate threat to my life or, if it seemed reasonable that he was about to cause me serious bodily harm at that moment (such as if he waved the knife menacingly or thrust it out toward me within close proximity; or if he was within about 25-30 feet and if he tried to get up with knife in hand, or made as if to throw it at me or my wife, I would not hesitate to shoot him as I moved to cover as long as my wife was not in the line of fire and; of course, I would shoot to stop. If he died as a result of my shooting to stop, which means I would have been shooting at the best available target area such as a possible center mass double tap and one to the head type thing, well so be it. I did not instigate the affair, he did. I simply defended my life.
Bear in mind that was just all my opinion. If you want to keep a gun in your home for self defense, get some decent training on how to use the gun and on how to act in life threatening situations. Then practice it again and again.
Safe shooting,
Glenn B
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. i am one of the niceset guys i know until you threaten me or my family, ESPECIALLY my family. in a bad situation i will very likely risk my life for you and not even have to think about it. but you come in my house and theaten my family and i'll see you in the next life 
