He was leading the Sith, yes. But under him, they were not the same as they were under Malak, which you don't really see in the game; I guess it'd be more of a lore thing.

After the Mandalorian Wars, he said it wasn't over and the Republic needed to prepare for something much worse. No one listened to him in the Republic, and the Jedi council all but banned him for fighting in the Mandalorian wars in the first place. So he did the only thing he thought he could to save the galaxy, he turned everyone who was loyal to him (millions of soldiers, and thousands of Jedi) to the Sith so he would have an army capable of stopping the threat. Under his lead, the Sith did take over planets to tactically control them...but that's it. Yes, he killed the forces going against him, including Republic soldiers; but never citizens unnecessarily. If you just automatically see the republic as "always 100% right and good" then yeah, he was the enemy. But to him, he was the good guy, because he was going to save the galaxy from destruction. I would argue he wasn't light or dark, really just gray; like a great military leader would need to be.

After he loses his identity; the game (because it's a game) kind of forces you to turn him completely dark or completely light. But that wasn't Revan.

As I said, Malak though, once he took over, the sith became very different. They got much harsher and started killing for nearly no reason. So you might think, because of how bad they were under malak, they must have been as bad under revan; but that's wrong.

If you notice, he also never attacked the Jedi Order, they sent Bastilla's group to attack him, because so many Jedi were agreeing with and leaving; and he was just going to defend himself when it got to the absolute need to...but he never set out to kill Jedi.