Who said anything about innocent people?

is money involved?

well, I did..... Serial Killers always kill innocent people.

Besides the fact, KILLING IS BAD.

is it?

besides, there are exceptions to every rule.

depending on the circumstances

I'd talk to them, find out the situation, and ask them what they wanted to do about it.
If it was justifiable, then I'd suggest taking it to the police.
If not...but they still had a reason, a reason that I considered good enough, I have to say that I'd suggest we moved someplace else and try to leave it behind.
If it was just like "I just felt like killing someone", at that point, it would be a problem. I'd tell them they should give themselves up.

I'd buy her a copy of American Psycho as a manual for better performance.

I think it would depend on the situation... who it was that they killed, and for what reason they killed them...if it was for revenge or another good reason, I'd probably keep their secret... but if it was just some random person for no reason, I'd pretend to be understanding, and then turn them in the first chance I got, because I'd be scared they'd kill me next. :p Actually depending on my mood, I may not care whether I was murdered or not.

I'd make her give me money for not turning her in...then make sweet love for the rest of the day :D

we'd care, goddammit !
:D