An ounce of prevention, here, is worth many thousands of pounds of misery later.
You literally think there's some likelihood that you will have to SHOOT your brother-in-law? So his "scumbag" habits aren't simply theft, or drug possession, tax evasion, jaywalking, or hanging with the wrong crowd then. You literally feel he will physically attack you and/or your wife in your home?
You need to be doing absolutely anything and everything to end the situation which is leading you down that path. Reduce or eliminate his visits. (Don't have to be confrontational about it. Lots of ways to be unavailable, not convenient, got other plans, etc.) Get your wife involved in this discussion. If he's that dangerous, she should be able to recognize that and help you reduce your exposure.
And when he is around, everything of value under lock and key, except for a carefully concealed weapon on your person.