Ok, so I'm hearing some say Mace is a bad guy, with lots of good reasons. Others say no because there are already two Sith, no more, no less.
How about this: Mace, seeing the decay of the Republic and the corruption it brings, desperately wants a change. He knows the Jedi Order as a whole will not support the dismantling of the government, no matter how noble or necessary it is, so he takes it upon himself to save the order and the galaxy by aiding in the effort of the separatists, and therefore Dooku & Palpatine, who tell him they only want what he wants, which is order in the galaxy. So he provides the "cloudiness" of the force, the erasing of the databanks, etc.
Now I admit that Mace has to have some aggresiveness/power hunger/dark side in him to do all of this, but it doesn't mean he has to be a Sith, keeping to the two-only rule.