One thing you have to take into account, is what causes the murderer to murder someone.
In affect-related killings (which I believe is a large part of all killings) the killer will not have time nor capacity to think "what will happen to me afterwards". This is why the punishment plays little role on how hard you hit your cheating wife/husband (or whoever).
The punishment for the crime might, however, influence a delayed reaction (a planned killing).
Note: I'm not saying that either is okay. But I do believe a planned killing should be punished more severely than a reaction (depending on what caused the reaction).
I can understand Magyk's point about prisons looking like retirement homes. This is indeed a problem, but only for those who go in and out again and again.
What really works is education. Those, in Denmark, who go to prison and take a degree, get out of prison a better man, and is much less likely to go back. Those who do not, have an increased risk of going back to prison, compared to the general population.