Viktor Accepting His Own Doomed Mortality After Sky's Death.
Sky tries to tear Viktor away from the Hexcore, but he is either unable, or unwilling (perhaps both) to step away.
She is turned to dust due to not having shimmer in her system, but I'd also argue it's because the Hexcore isn't interested in a third wheel in its situationship.
Viktor calls her 'Sky' after her death, the first time we've heard him say it. Previously, it's all been 'Miss Young'.
Guilt, much?
Guilt and grief.
He understands the Hexcore needs to go, but also what this means for him. No Hexcore = no cure. He tries, but can't do it.
Viktor turns away, and the Hexcore either punishes him for his action, or enacts self-preservation. Regardless, it's making its own decisions and putting them into action.
Viktor heads to the overlook, and contemplates stepping off the edge.
Jayce turns up, and stops him. This is after he's just killed the child, so maybe he's up here for his own spiral? He agrees to destroy the Core, he promises Viktor, but doesn't do it.
Viktor fights to live, until Sky's horrific death. Guilt-ridden, and seeing his creation for what it is, he chooses not to go any further. That's until Jayce happens.
Later, after Viktor sets up his commune, Singed turns up, keen to get his hands on the apex version of Hextech, and to combine it with apex shimmer. Anything to bring his beloved Orianna back.
Singed points out that Viktor's power is dwindling, something Viktor is already aware of.
By telling him to leave, Viktor is rejecting Singed's beguilement, which means he understands that (once again) his time is finite.
That is, until Jayce happens. Again.
So that's twice Jayce could have stopped Viktor from becoming the Machine Herald, and twice he helps bring about its creation. Literally forging Viktor into a weapon.
This makes no sense for the story if Jayce's whole arc is simply bringing about Viktor's demise, particularly when combined with, 'all I want...
It only makes sense if this is the intended destination all along, that Jayce's plan has been to save Piltover (and by extension Zaun) by defeating Noxus and the evolved, but also get his partner back. Ergo, his conversation with Mage Viktor must have been a lot longer, and covered a lot more, than we see. Jayce's goal was to get to the top of the Hexgate, where he has already seen his alternate version, and then speak to Viktor, and try and talk him down from the metaphorical ledge.
During the battle of Piltover, the Machine Herald rocks up to the Hexgates, whilst everyone apart from Jayce is elsewhere, having pulled a sneaky bait and switch.
Jayce doesn't look one bit surprised, although he does look determined.
Cue big fight and flight, then Jayce takes a deep breath, and shuts his eyes, as he kneels into position. Showtime.
The only flaw in this whole plan is that Jayce needs Ekko to Z-drive the Herald into oblivion. Perhaps this is something MageVik doesn't actually know/understand, he's just busy trying to find something that works - that stops him ascending to godhood, destroying Piltover, Zaun (and possibly the world), but also brings Jayce and Viktor together.