It is not as simple as the OP thinks.
Upping the punishment significantly for crimes where nobody is seriously injured or killed to the same levels as those where someone is has the effect of giving criminals the all or nothing choice.
If they are going to commit robbery why stop there if the punishment is the same for robbery as it is for murder, and murder leaves fewer witnesses?
You have to give significantly lower punishments for the less severe crimes to retain deterrent for crossing certain lines.
So you must assign a punishment for the absolute worst of crimes, and everything else must then scale back from there.
You want to retain a system where the criminals realize there is additional boundaries they don't want to cross even after they have already crossed one.
They may have just committed a robbery and be fleeing, but should still be reluctant to kill for example.
Each such boundary reduces how many take it to the next step.
There is not much of a difference between 25+10, 50 years, 75 years, 900 years, etc to a criminal who already thinks short term. They all sound like "life" and likely would be life. So you have to start your average for the worst things lower.
Then each lesser thing must be less time than that, and the end result actually needs to be that criminals who do less horrendous things get out sooner.
The problem on the other end, starting minimum sentences too high is that once you actually send a criminal to prison for a long time they are far more likely to remain a criminal for life.
Bad choices among teenagers and young adults is high, but that rate quickly goes down as you get into older age brackets.
But that is not the case with a prisoner that has spent the last decade in prison surrounded by other scumbags. They may have grown out of it, but by living that lifestyle they are far less likely to.
England tried dishing out the harshest sentences for most crimes, and it didn't work. The tiny island of England had so many prisoners they populated new continents with them.
Various nations still operate that way. You can lose your hand in various Muslim nations for theft, be stoned to death for admitting to cheating (a crime) and similar punishments that were once popular in western culture.
You don't want to create more "all or nothing" criminals by having all of them facing life sentences the moment they cross the first line. That just increases the viciousness of your typical incident and their intention to get away at all costs.
Instead of giving up to face their 5-10, they kill and fight to the death to escape their 25+.
We see this with the three strikes law (and the third strike can be something relatively minor, like drug possession.) Some guy that should not have gone nuts suddenly poses a lethal threat to citizens and officers because they are facing a life sentence, and neither officer nor citizen expected it.
Longer prison sentences do reduce violent crime. It's not so much the deterrent factor but the fact that it's hard to rob and kill from an 8x6 concrete cell. The vast majority of violent crimes are committed by a small number of people, so locking them up keeping them otherwise engaged.
There is some merit to this, but at the same time we are supposed to be a free society. In a time when prisons are being privatized and prisoners are now a business and source of funds and labor, there is incentive to keep a steady flow of them coming in.
Some in the private prison businesses have been the strongest financial backers of various legislation to increase sentences in several states.
If you have a huge industry just waiting for citizens to step over the wrong boundary to become a slave, and that industry working hard to increase penalties and the number of crimes, you cease to really be a free society.
Freedom just becomes an illusion for the percentage that have yet to cross one of the ever increasing number of legislated boundaries.
There is no perfect solution, there will always be a percentage of violent criminals in a free society. That percentage will be greatly reduced however if the risk of serious injury or death is high in that line of work. (Places with a lot of armed citizens often have increased property crimes rather than violent crime against strangers as criminals are more likely to avoid armed citizens.) It will both deter as well as result in fewer surviving, without disrupting the intended tiered deterrent of criminal punishment by the legal system.
I am all in favor of an armed society and a lot more criminals shot during the course of violent crime. But once they are caught or surrender you have to give them an appropriate sentence proportional to the crime.
One of the issues that I see is that we have SO many ways to get locked up. Violent crimes deserve just punishment, don't misconstrue it mean anything other than that, but some laws are simply outrageous. We have so many laws now that it's impossible for anyone to know them all. 
If they want to keep the guns out of criminals hands, let's start by NOT making so many felons. They should help repeal laws that punish felons with restrictions on firearms for non-violent and non-firearms related crimes. I'm sure that they would also help lobby to strengthen existing laws to keep violent offenders off the street altogether. Anybody that truly wanted to keep guns out of the hands of criminals could start there.
