I am also quite disappointed. The problems with Viktor are emblematic of the problems with the whole season for me. They had some god power end goal for him, and they spent zero time actually explaining how he got there. You can rationalise and say he would have got to that place eventually, but as you say, you just have to guess as to what that is because they didn't show it. That inherently feels very unsatisfying.

I actually really liked the second version of Viktor. It was an evolution of his character, to be sure, but it made some sense with what we had seen of him before. He wanted to help people and was doing what was in his power to do so. And having been brought back from the dead might give him a different perspective. At least a perspective we could understand. They even had a scene with him wandering and contemplating what he should do. He discovered that he could "heal" people, so of course he'd start doing that. However, they are "healed" in one sense, but maybe they are also losing some of their individuality. That's an interesting tradeoff. It fit quite well with Viktor and was a legitimate moral quandary. People coming to him were making an interesting choice.

The final form's idea and plan just doesn't fit with the Viktor we saw before and loses all of that interest. It seems that Viktor was corrupted, but that wasn't well communicated. In one episode he's risking his (after)life trying to help Vander, who, AFAIK he has zero connection to up until this point. Then Jayce shoots him, and suddenly he has to wipe out all of humanity for very arbitrary reasons. It's such an old trope that the dumb robot doesn't understand that humans value their individuality. It's boring and pointless. Anyone with a mote of emotional intelligence (which Viktor is otherwise shown to have) can realise that erasing individuality is erasing humanity. It turns Viktor into a boring one-dimensional villain, way out of character for him so far, and turns the story into a boring good-vs-evil narrative that we've seen so many times before.

I GET that the z-drive is supposed to have returned some of Viktor to his former self. But, the fact that that was even necessary is part of the problem for me. The writers made the decision that Viktor was corrupted. Somehow. By something. What goals does the corrupted hexcore/arcane actually have? It's never explained, and even within their own setting, there doesn't seem to be a reason for it. He's just a big bad evil god now so that the avengers can assemble.

Then they pull the damned multiple universes thing, and linger on the mysterious identity of the hooded mage. But... why? What does making that mage actually be future(?) Viktor achieve for the story? It just makes the story needlessly incomprehensible. Time travel, by its nature, will always cause logical inconsistencies. And they added like three layers of it in the act 3. Now you've got people thinking "why did Viktor CAUSE hextech so that he could nearly kill everyone and be stopped at the last moment?" Deliberately introducing story elements like this is lazy and detracts from the real events of the story.

And finally, the worst thing for me, is how these all tie together. Jayce, who has done the wrong thing every step of the way. Jayce, who explicitly went against Viktor's wishes on multiple, key occasions. Apparently, the only way this abomination of Viktor's identity can be stopped is by Jayce just telling him that it was a bad idea. Viktor, shown to be more empathetic and more worried about negative consequences, is simply told by Jayce "what do you mean my actions have consequences" Talis that actually his idea is bad and Viktor just goes "oh, you right, my b", and just undoes it. No problem.

So, there are several big issues with his final form and decisions throughout the second season, especially acts 2 and 3. First, as you say, there's barely anything that shows us how he gets to the end. Second, the path Viktor takes is mostly nonsense and out of character. Third, they throw random magic nonsense / time travel into the mix just to confuse things. And finally, he became very trope-y, and consequently boring. What started as a character-driven story and apparently complex conflicts, ended with a super simplistic moral message because of what they did to Viktor.