Tons of action movies beg to differ. John wick is a good example. Plenty of actions scenes and justified killings, but the story isn't about killing people it's about John Wicks escape from his bad past and trying to get out from it. The amount of killing is there to ramp up the stakes. John Wick deserves to be hunted since he was a contract killer but still people cheer for him because the story isn't killing, it's trying to let go of ones bad past.
It doesn't have to be justified... Sylv can be redeemed but people can still say a genocide was the bad course, it CAN still be the bad course. Doesn't change the fact that it can be understandable with their reasoning.With killing it's a bit different because people find it way easier to justify killing people e.g. in self-defense, as an act of war which of course makes sense but even then, there's a point to be made about the glorification of violence that is often involved in showing the act of killing.
With genocide it's totally different. Most people can't even conceive a good enough reason that would justify something like a genocide. Again, this is not about not depicting a genocide or telling a story about it, it's about telling the story in a way that makes genocide the only "right" course of action.
It's like the AI trope. Who tries to make it better for humans and they conclude that killing humans is the best because they kill themselves yadda yadda. It's obviously retarded and not justified to kill humans due to that, but it's still understandable when you put it in an objective view. I know what people go for when they tell that story.
Another great example is Sargeras. He literally tried to kill Azeroth and that would include everyone who lived there which would be genocide times X. Do I understand his motives? Absolutely. Do I understand his reasoning? Absolutely. Do I celebrate his actions or think they are the right course of action? Hell no. Is the motive noble in the greater scope of things? Sure.
They don't HAVE to make the genocide the right course of action just because they decided that Sylv tried to do something noble and justified to her.