I agree that the handling of Jayce attempting to kill Viktor should have been better, which was likely sacrificed in favor of shock value, but I still don't see why the idea of killing Viktor seems out of place. He just spent weeks alone in the apocalypse he helped cause, and the moment he is sent back in time, he choses the nuclear option of ending the threat as soon as possible. Hextech is evil, Jayce won't make it anymore, and the only other person who can is Viktor. Viktor doesn't want to stop, so goodbye Viktor. What other plan could Sorcerer Viktor give him? His timeline is different from Jayce's, the only thing that saved them was Ekko's anomaly that Viktor thought was impossible.
Also, if you rewatch the scene, it is most definitely not "without a moment's hesitation". I mean just look at him, it was clearly not an easy decision, nor does he look like he is thinking straight.
I think the real problem here is that we don't really know WHY Hextech in general, is so inherently wrong. It just fucks things up, I guess??? If there were some sort of principle behind it that Viktor agreed with but Jayce did, it would be easier to swallow. Viktor's pursuit of perfection is what led to his downfall, but what would have happened if Hextech wasn't some ticking time bomb?
Anyway, apparently Mel is being added to LoL as a playable character soon. Which is crazy, because that means her whole reveal this season made sense to no one. I assumed it was some surprise easter egg of sorts for the players, but nope. She's an Arcane only character.