Throughout season 1, Viktor's journey was one of isolation. He was an isolated child, an outsider in Piltover, met Jayce but then grew apart from him throughout acts 2 and 3. As the Machine Herald, however, he's connected to everyone.
"I'm alone no longer..." he says in the Council Chamber...
"I now speak with all their voices." And I think the camera move is really interesting, the way it spins around him, it's like it *should be* a reveal, you would expect the camera to whip around and show all the voices Viktor is speaking with. But instead, all it reveals is that he is alone. It's a false connection, but he believes in it.
At the Hexgates in the finale, Viktor tells Jayce about what he thinks he's doing. He tells him that he's setting them on a path where "we can be of one mind. United."
Later, when Viktor brings Jayce into the cosmic realm...
"This is what you see?" Jayce says. He's been fighting VIktor since he got back, but now it all makes sense. All these souls, together, and connected. And it's beautiful.
In the end, though, Viktor is absolutely shattered by what his older-self says. And why? On the surface, it's a calm scene. a beautiful scene even...
But dreamless solitude... Old-Viktor isn't just admitting he was wrong, he's saying he created a hell for himself. He condemns himself to loneliness. And it destroys him...
I also want to take an aside, to talk about the other part of what Old-Viktor says...
This ties into something Machine-Herald-Viktor said earlier, and contrasts with it, that he thought he was heading down the path of "an unbroken saga of progress..." He didn't realize where he would end up was "an end to pursuit." And what is an end of progress, and end of evolution, if not death? Viktor had condemned not just the world, but himself to a waking death. But that's a tangent from the theme of isolation vs. connection I'm focusing on here.
When Viktor comes back to himself now, he sees all the "evolved" souls differently. This wasn't connection that persisted until he was old, it was a false, forced connection. He is actually alone.
Except that he's not alone. It's Jayce's love for him, that real connection, that loves him despite everything, that he didn't have to be a saviour or a herald to deserve, that saves him.