I'm finding it's fairly pointless to kill zombies as Nemesis for mutation.
With the change from PTB allowing you to gain Mutation from hitting contaminated survivors you don't really need zombies for mutation.
The only time it seems like a good idea is if they get stuck to hopefully reset them.
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I only do it when it's almost leveled up and there's no survivor anywhere to be seen but there's Stacy stuck on a rock, so might as well.
There is an addon to get undetectable after killing a zombie.
Which is much better. Survivors shouldn't have that much control over a killer's power. Generally you want to kill them for buffs (if you have addons for it) or to get a quick boost if you just barely need to tier up.