Strangely, I'm not really that invested in whether or not it was wrong not to kill Genocide.
My point is that I can sympathize with why she didn't kill Genocide. It's like if I kill a spider or some other creepy-crawly bug that I discover in my room. (Obviously, the fear of a bug crawling on me without my knowledge is not equivalent to the danger of Genocide hurting, killing and torturing people, but.) When you grab a tissue and go to squish that bug, there's something horrifying about pitiable about it. I feel guilty that I'm killing it, horrified that it might touch me, feel sorry for it if I don't kill it right away and its legs are twitching, and feeling doubly guilty that I feel it's necessary to kill it in the first place.
If you anthropomorphize the spider, you will feel sorry for it. And for Wonder Woman, knowing that Genocide is, in a sense, her, or a possible-her, it is very understandable why she would flinch at the last minute and not squish that bug. That response makes even more sense when you take into account what she said in this issue, about it being jealousy that motivated Genocide against her. She was an awful, pitiful creature who had nothing, and could do nothing but destroy. It's hard not to be revolted and feel pity, and action based on those feelings is to stop and let go, rather than to finish the job.
I can understand and appreciate the reaction that she had, even if she reasons it out differently in her own head, with talk of vengeance vs war.