Even for those extreme cases, it's understandable to wish for that, but I'm not sure it's healthy for the one wishing. Speaking only of people who don't believe in the death penalty, and are breaking their moral code due to an extreme aversion, maybe it is healthy as it may be cathartic; maybe it is not as it may reinforce rumination, stressful feelings, etc. Maybe it is healthy as they can reach a slight feeling of justice or equilibrium again in the world; maybe it is not healthy because they'll feel they themselves committed a moral transgression pushed by the atrocities of these people. I don't know, maybe it's different from person to person.