Yeah but your missing the part where....Revan...is...the...only...Force-user....to master BOTH light and dark.....well...Till Cade Skywalker but Cade doesn't "like" either Jedi or Sith. Revan is still considered a Jedi Master, and for the most part fallows the Jedi way. Mace Windu even tried to wipe out the Sith. No Jedi want the Sith around.
Problem is, DARK SIDE does NOT equal SITH and SITH does NOT equal DARK SIDE. He wanted to WIPE OUT ALL SITH. GENOCIDE. Very very evil thing. Very very dark side thing. Doesn't make him Sith. So, if anything, if he DOES return, he'll be on his own as a fight for both sides to stop him from taking his "Crusade" too far. A Sith can be light side and a jedi can be dark side. There are also many force users, on both sides of the force, that aren't Jedi or Sith. Teras Kasi, Sages of Baran-Do, Nightsisters, Witches of Dathomir, Sorcerers of Tund, the iist goes on. Some of those are dark side, but they aren't Sith. Some of them are Light Side. But they aren't Jedi. All of them can "fall" or "rise" the same as a Jedi or a Sith.