Being against death penalty, but pro assisted suicide, is like…what is the analogy?

One can say no to death penalty for even serial killers, terrorists, for example, but be pro assisted suicide? Helpless patients whose only crime is to be in pain, or be pressured into wanting death by their greedy family for example, are given death instead of pain relief, all in the name of mercy.
What could be the analogy? As in it is far from consistent.