Imagine you had the power to control time. And something bad happened, like somebody's death or something. Normally, this is how things go. You'd make sure it never happens again.

But this time, you reverse time to the point where you could stop that death from occurring. And once you do it, everything seems normal (at the time).

If there's anything you need to know about tampering with time, it's that there will always be consequences. If this wasn't the case, then we should reverse time back before the Avengers, contact Thor, tell him his brother is evil, have him locked up, and save a lot of lives.

Baron Mordo has always been the bad guy in the comics, in this one he's on the good side in a bad way. What he says is right (about the tampering), but even his plan is flawed (about taking down all sorcerers to prevent that).

Learn from The Flash. Look what happened. Or the game called Life is Strange. Murphy's law states that if anything can happen, it will happen. In this case, something worse than Dormammu's invasion will happen (like Thanos', is what I believe, with his Infinity Gauntlet)

So, yes, Mordo may be evil now, but he's tryna do what he thinks is right, so that balance isn't tested with again, even if it seems all villainy and all.