One of the reasons why there's gradated punishment is to try to provide some disincentive to carry out "worse" crimes.
In your example, if I'm planning to steal from a little old lady then I may as well kill her in the process because the penalty's no worse. That's of course if you view this kind of punishment as a deterent. If it's just retribution then attempting to apply any kind of rationale to it is, I concede, pointless.