I'm actually going to try and defend Jayce here. I'M GONNA WRITE A LOT FAIR WARNING.
It is difficult to find a champion more reviled by a lot of the Lore boards than Jayce, and the worst part is that he doesn't deserve a lot of it.
Jayce is, personality-wise, an arrogant dick, yeah. However, Jayce is still working towards bettering society as much as he can. The popularity of Viktor and the unpopularity of Jayce have sorta created this narrative of "viktor is the true hero of the story and bad man jayce is covering up the truth!!!", but it's more complex than that.
1. Viktor Is Not Perfect
I think Viktor is one of League's most interesting character's on a meta-narrative level, because it's obvious he was originally meant to be a villain. His design, his splash, and his in-game VO and ability names all reflect that. Viktor casts his E, "Death Ray", and then cackles. As part of his core gameplay loop. However, Viktor's release lore was extremely sparse (as were a lot of champs of that era), so many fans got attached to his character and fanon made him an anti-hero or misunderstood figure. With the rewrites of 2017, that became canonically what Viktor is. But we still can't forget a few things.
He devised a chem-shunt helm that allowed an operator on the surface to bypass the wearer’s fear response and, effectively, control the diver.
Viktor tried to make a suit that would effectively allow someone to enslave the wearer. You can argue that it's probably a consensual process putting the suit on, and that the diver is aware of the risk, but look at the way Piltover/Zaun is structured in the story. It's effectively a Class Divide Utopia, and it's not going to be the affluent Piltovians in the suit, it's going to be the desperate, poor, nowhere-else-to-go Zaunites who take on that responsibility.
**2. Jayce Is Not All Bad
**
Straight up the first paragraph of Jayce's Lore:
Still, Jayce’s heart is in the right place, and even those who envy his natural skills are grateful for his protection in the City of Progress.
Jayce has devoted his life to protecting citizens of Piltover (though his views on Zaun are likely pretty shaky). His attitude is off-putting and his demeanor is rude, but he still wants what's best for everyone. His conflict with Viktor isn't one of hero against villain, it's two people trying to do good...with vastly different ideas of how to do so.
3. Their Incident Is Based On Misunderstanding
This is one of those things where reading one of the lores alone will trip you up. It takes reading both Viktor and Jayce's Lores to get the full scope of the Crystal Incident.
From Viktor's Perspective:
- Viktor watches hundreds of people get fatally poisoned. In a rush to save them, he builds a machine to cure them by opening their heads and sucking the poison out. However, he doesn't have enough energy, so he has to go to Jayce's lab and get his Hextech Crystal power source.
-Viktor explains that he needs this to save people. Jayce says no, and he was always kind of a jerk, right?
-Viktor steals the crystal to save hundreds of people, but he has to transfer some of their brains into robots to keep them alive, because their bodies are too degraded from the poison.
-Viktor returns to his lab to use the crystal when Jayce arrives, mad and with a big ol' hammer and begging Viktor to stop. He tells his guard robots to kill Jayce. Probably not something he wants to do, but he can't afford to stake the lives on this.
-Viktor's warehouse is destroyed by Jayce, the people die, Viktor considers Jayce another example of humanity failing.
From Jayce's Perspective:
-Viktor shows up on his doorstep. Keep in mind that Jayce knows nothing of the poisonings or any other emergency, just that his old colleague who tried to build a mind control helmet is here.
-Viktor invites Jayce to join him in...well...this:
He’d finally learned how to achieve his dream – how to eradicate disease, hunger, hatred. If Jayce joined him, the two would accomplish more than anyone from Piltover or Zaun could have dreamed of: they’d save humanity from itself.
Jayce has probably watched a hextech movie or two where the villain says something along these lines. I know I have.
-Not trusting what the guy who frequently breaches ethical rules will do with a superpowered crystal, Jayce refuses. Again, it doesn't sound like Viktor ever actually told him what was going on.
-Viktor beats him up and steals the crystal.
-Jayce hunts Viktor down and enters the warehouse Viktor is working in. He sees dozens of seemingly dead bodies, with their heads sliced open, next to a bunch of replacement robot bodies. He knows Viktor has always wanted to start a "Glorious Evolution". ** He has no idea these people are going to die, or that Viktor did not kill them**.
-Jayce pleads with Viktor to stop. Viktor tells his guards to "Kill him".
-Jayce brings the warehouse down, stopping what he probably thinks is a mad scientist creating his own robot army, which has just tried to kill him. This is the version of the story that gets out, because Jayce is the more socially powerful of the two.
In the end, I think the biggest failing in their fight is that Viktor did not simply tell Jayce exactly what he needed the crystal for. Both of them are too stubborn to really listen to one another (although admittedly Jayce tries towards the end).
Basically, Viktor and Jayce want the same thing, but Jayce has lived on the posh side of humanity and believes that humans have infinite potential. Viktor has been kicked around by humanity all his life and believes that humanity is what holds people back.
They're both idealistic young inventors, and I don't think either can really be called a true "villain".
tl;dr: Viktor and Jayce are both good people at heart who are too rolled up in their own ideologies to ever compromise. Viktor is more popular so he is viewed as the "good guy", but both of them have good/bad qualities that pretty much even out.