[RedSillyDragon]

With all the time traveling it might be more to it than just Oma Zi-O being the big bad guy. Sougo having that dream, could be from another timeline. Like how the person having dreams of (true) events from an other timeline. But then how many layers of timeline has been changed. Few games I played follow that trope of "What you dream may not have happened in your life, but it did at one point in another timeline". So we have...

Current Sougo is having dreams from alternative timeline.
Oma Sougo had the same dream from it but it lead to him being an evil overlord.
Dream Sougo of that timeline witness it happening for real.

This would mean there are at least 3 different Sougo's, if the dream never happened in their timelines, but it did happen before it got changed\leading into Oma Sougo Timeline. Just like how they warned Current Sougo about messing with timeline. Someone did messed around big time with Dream Sougo Timeline.

Now the curious question is who did Dream Sougo spoke to? It seems someone from the future trying to set him to a path(unless that was yet another Sougo.. the 4th one). Mentioning, how he would save the world. So if to say the big plot twist would be the true villain was someone else but instead of 50% population wipe, it was 99%-100% of the population wipe. That means the 50% population wipe would be saving the world but in the wrong way.

So what made Oma Sougo decided to go that path? Perhaps he got scared that the true villain is hidden among the population, that would rise and go for 100% wipe. So the only way to encounter it, was to prevent it by going for a evil overlord. Or perhaps Oma Zi-O got brainwashed or possessed by something. Well just feel like throwing crazy theories around xD