I don't buy "Don, the slavemaster." We've never seen him treat any of his other (many) women so badly. About the kinkiest we've ever seen him was telling a hooker to slap him...but that's not really the "master" side of that equation, now is it?
I looked to me as though Don--after learning that his dalliance with Sylvia was leading her toward throwing away her marriage--set out to purposefully send her running back into her husband's arms.
That would, on the surface, seem to be a rather selfless act on Don's part...except that I've never really seen Don as someone who actually gives much thought to anyone else's well-being. I think it's probably more likely that Don just wanted to get clear from all the drama and inconvenience Sylvia's escalation of their relationship threatened to bring.