Thing is, it's incredibly problematic in light of Diana's internal monologue earlier in the issue. She asks herself if she hasn't been self-serving and self-righteous in not killing her foes; if perhaps she's been valuing her own self-satisfaction and ability to say "look how virtuous I am" over actually making the world a better, safer place. It's an extremely legit question. It is a question that needs an incredibly compelling answer. How can Diana continue to be considered a good guy if she can come up with no answer to "why is the one life of a villain worth more than the countless innocent lives this villain's existence will cost" other than "because that way I feel better about myself"? She absolutely can't. It completely condemns her behavior. Once she asks herself that, once she puts that incredibly morally powerful idea on the table for herself and the reader, she cannot spare Genocide's life without proving herself a monster unless she can offer an equally morally powerful rebuttal.
So what is her justification? What compelling reason does she eventually come up with to answer her own self-doubt, to rebuke the question "what genuine moral good am I serving when I choose not to kill a foe?"
All she offers is "it makes me feel better." She can't do it because she doesn't want to be that person. That's all. That's exactly what she condemned herself for, exactly what she accurately said would not be morally acceptable, half a fight scene prior, but it seems to be enough for her, since it's her final word on the matter and she limps her way home to Themyscira totally satisfied.
In the absence of that monologue, sure, not wanting to kill Genny because it feels wrong would be perfectly acceptable. But in the presence of that monologue, with that awareness that "feels wrong" is not a legitimate justification in the immediate forefront of both her brain and the readers', it becomes monstrous. It's the difference between misreading a label and seeing the dosage just fine but still handing out too many pills - awareness is everything, and Diana is incontrovertibly aware.
(Ideally, she'd have killed Genocide, like she damn well should have and promised to a dozen times, and thought to herself, "no more. Not ever again," and gone straight to where the Cheetah was holed up. And then something gets said or done to remind her of when Minerva was her ally, and she thinks to herself, "wait, that's why. Because they can change. Because I believe in reformation, because I must not prevent the good someone might do if given the chance." Because that question cannot go unanswered like this once asked, not without making Diana look like the worst kind of hypocrite.)