Hmm. Okay, how's this: once Genocide had the Lasso removed from her, she's a lot easier to handle and defeat or imprison. She could theoretically save her, and then lock her up somewhere, right? IDK.
Again, I can understand the motivation in not wanting to kill her, even from the vengeance angle to an extent. If doing it feels wrong in the moment (and I think that we do base a lot of our morals on how doing something feels or how the consequences would feel, which in turn explains why some people do horrible things: they somehow get to the point where doing something hurtful actually feels good) then of course she will question why she's doing it and whether or not it's the right thing to do. Killing Max Lord probably felt right in a way that–to use the example again–killing Frankenstein's monster never can.