Jayce killing Viktor seems to serve no purpose. All it does is speed up Viktor's transformation into the Machine Herald. Except...
Understandably, this act of violence puts the two partners indisputably on opposite sides to each other. It's the antithesis of renewing wedding vows, and instead is a visceral resigning of their divorce papers.
Unless they are on completely different sides, Jayce will fold like an origami frog, and immediately he does so, there is no one to get through to Viktor (Viktor, rather than being all free of emotions, becomes decidedly touchy and responsive whenever his partner's back). Jayce has to be himself for long enough to make his speech at the final battle (also gives Ekko and au!Powder time to make sciencey things), and he can't do that if he's having fun at Viktor's commune.
Once Jayce has done the deed, he seems to come back to himself, albeit with post-ravine trauma, but I think that's the point. He had to be all kinds of fucked-up to pull the trigger. All it took was a very nasty fall, a severely broken leg, raw salamanders, horrible, scuttly mannequins, an enormous climb, hallucinations, hopeless solitude, and being pushed to the edge by another Viktor. Oh, and seeing a weird, maybe-not-completely-dead version of himself.
Also, I've just read the lyrics to
and it's made everything even sadder... ☹️