But what would be the point of that?
In the end,
Malgus fails. Or at least that's what we know. So you would still end up abandoning rank among the Sith, albeit forcibly.
Well,
Malgus fails because you and your group break into his place, trash all his stuff and kill him. If you hadn't done that... he's got a super-powered stealth armada with alien troops and tech equal to or better than the Empire's stuff, you know.
If you had joined him instead, he'd be sitting pretty well, I think.
But defecting from the mainline Sith Empire would require a new spacedock, probably. Bah.