My fortress characters have not killed neutrals with a couple of recent exceptions. The recent exceptions were because I finally gave up on fortress living to the ideals you'd expect because it was manifestly incompatible with most people's fun stick.

It's probably no coincidence that my last fort character was also my first to accidentally kill a good aligned mob.

If nothing else restricting your killing of npcs to evils does minimise the risk of a tragic mistake.

I'd happily kill neutral pcs though if they were doing evil stuff at the time.