Do both: dump him AND get revenge.
I once had a boyfriend who said i didn't trust him unless I gave him my password for Hotmail/MSN, so I gave it him, and guess what? He CHANGED the password, told me someone had hacked in and told him they were someone I was cheating on him with, AND emailed all the male-sounding contacts with "who are you and why are you on my contact list?"
I found this out because the dumb-ass used one of my old passwords as a replacement. I rang him at work and told him I managed to get back on, and "wtf", basically. He denied and denied, and got quite irate - "I can't believe you'd think I'd do such a thing.. why don't you trust me" - and rang me later on to ask me if I believed him. I gave up, and said I did (even though I didn't, but as I was living with him at the time and felt trapped in the relationship, it was the easiest option), and the tosser admitted it! So he tested my loyalty, wanted me to believe his lies, and humiliate me by telling me I'd done that. God I wish I'd smacked him when I had the chance.
If he can do this to you, and you let him get away with it, BE SURE that he will do it again, and possibly more.
He did a lot more privacy invasion, but I thought this anecdote was topic-appropriate.
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We are the mutant race!! !! Don't look at my eyes, don't look at my face...