Frankly, I do not believe that once a criminal, particularly a violent one, has reached adulthood, that there is much liklihood of reform, at least until he is much older (like at least 50). Once his psyche/moral system has formed to a certain degree, it is very hard to break. If he is basically honest and peaceful, then great. If not, we are in for a lot of trouble until he gets too old to act out or just loses the compulsion (or is dead).
I know everybody can cite at least one "success" story, but they are notable for rarity. Recidivism rates consistently run in the 70-80 percent range or higher, no matter what programs get applied.
I do believe that if you can intervene earlier, in the early and mid-teens, you have a chance to turn a kid around. A teen's psychological state is still malleable enough to allow for some large changes. This malleability can go either way --- he (or she) falls in with the gang/drug culture, he can easily be led to extremes, and we get these vicious kill-without-remorse types that scare the daylights out of cops (and everyone else). If he or she can be put in a structured setting, with close supervision and straightforward rewards and punishments, that same malleability allows him to be brought back. Normally you would depend on parents to provide this environment ("Train up a child in the way he should go, and even when he is old he will not depart from it...") but there are a lot of parents these days that can't manage their own lives, nevermind their children's lives.
There is a national non-profit organization (AMIkids.org) that deals with this, and they have had some pretty good successes. (I am on the board of one of its Texas programs). It started in Florida back in the 60s in Miami and focused on teenage boys, but includes girls now, and has about 50 programs in several states (including Texas). It takes teenage offenders, gets them out of the usual juvenile prison system, and runs them through a program of behavioral modification, study (towards HS diploma or GED), usually some vocational training, and some "life skills."
Some programs do slightly better than others, but the national average is 70-80 percent success rate. Success being defined as once graduated from the program, the youth does not re-offend within three years. If you know anything about recidivism rates for adults, you recognize this as an amazing success rate. Teens are screened for the programs, they don't just put anyone in it, but the teens are not all minor offenders either. Some do not graduate, but most do, and they usually stay out of trouble. It is pretty amazing to me how well the programs do.
But as I said above, I think the prognosis for an adult offender is pretty dim, regardless of what goodies you throw his way to help him change. Despite all the opportunity given, most seem unwilling to take them or stick to them.