RTF is a global modification that replaces all towns in the game.

Villains are traditionally associated with Darkness, so a distinction could be made based on this characteristic (without adding Chaos).

Yes, I don't disagree, but let's consider it specifically using races as an example:
Might and Magic (MX) - here, they are intended to be evil and love to destroy everything, so we won't touch them.
Orcs - Darkness - a characteristic of villains, and the second school naturally suggests Chaos (due to their connection with demons); Light is definitely not suitable, leaving Summoning, which is a questionable option.
Dark Elves - Darkness - a characteristic of villains, and they worship Malassa, which means Light is definitely not suitable. Chaos and Summoning remain. Besides Malassa, they also worship Arkat. Arkat -> Fire -> Chaos.
Or, here's another perspective: Light and Summoning
Naga - a good faction, so Darkness is out of the question. Their lifestyle is peaceful, and they generally like to create, so Chaos is also out of the question.
Humans - Light - it's obvious here. Darkness is out. Chaos and Summoning remain. Chaos shouldn't be given to them, at least because something like Earth Shock + a rain of arrows = death to all living things! Summoning remains!