I find it perfectly fine having the default morale penalty for killing innocents at -100 morale.
What I dislike about this, is that, unlike the morale penalty for killing zombie children, the morale penalty is consistently -100 morale. I would much prefer that for every innocent killed before the current one, the penalty would be reduced by 1.
So if you killed 5 innocents, your next kill would net you an additional -95 morale, and so forth. Sure, having to kill one-hundred innocents with guilt before you can do so guilt-free is a tad much, but it’s better than nothing, and keeps the Psychopath trait nearly as useful as it was before.
Alternatively, perhaps we could just apply whatever system is in place for zombie children to innocents. That would work as well.