First of all, we need to do away with the concept of "crime" when there is no victim. Then we can discuss what happens with actual criminals, starting with the fact that the threat of extremely severe punishment, like teh death penalty, doesn't seem to be a deterrent for the most egregious cases like murder and rape.
Discussion
Severity of punishment has more to do with retribution than deterrence.
More effective is assurance of prosecution. Which tends to be high for murder.
Rape prosecutions are usually he said versus she said, and a lot of psychology and culture, so I wouldn't include them in any sentencing debate.