Guys, why would he do this?

So, I was engaged about a year ago to a guy I went to highschool with and that I had dated off and on previously.

He was a cheater, is a cheater, and always will be a cheater. But, I loved him, and basically told him I knew he wasn't going to be faithful, so just to tell me when he did it so that I knew. It was better than being in the dark.

I put up with his bullshit time and time again, all because I felt like that was just who he was, and at the end of the day he was going to marry me, so who cared? Plus, due to the fact that he was in the military, we were apart a lot and I understood he had "needs".

So, all that said, I was finally financially ready to move from Washington, where I lived, to Texas, where he was, so we could get married. I spent $250 on a plane ticket, one-way, and quit my pretty good job, all to be with him. I told him I had finally gotten the ticket (he'd been begging me to get it), and he sent me a text saying he was being reclassed and they were moving him out of Texas.

"Okay," I thought, "I'll just go where they are moving him." But when I tried to find out, it was like he had dropped off the face of the planet. He would answer my calls, he would text me, he blocked me on FB... he was gone. I eventually moved on, mostly, but it still always hurt. It still stings.

Recently, I found out that he was "engaged" to at least 3 other girls from my city, and one of them still talked to him. When she found out I had been engaged to him at the same time, she made a fake FB profile and saw that he was also "engaged" to some girl in Texas who he worked with. (I found all of this out later, btw.) So, she sent the girl a nasty message, telling her about how he was with all these other women, and using me as an example.

Needless to say, when "Bob" found out, he was pissed, and assumed it was me. He called me up for the first time in ages and threatened me with legal action if I ever contacted either of them again, and then went so far as to accuse me of being the reason our relationship ended, accusing ME of cheating on HIM, of all things. (I assure you, gentleman, that was NOT the case.)

I somehow managed to keep my calm and explain that I didn't know anything about the FB thing, and assured him he would not ever hear from me. It took all my will not to tell him off.

Since then, though, it's been like an old wound was re-opened. Why would he do something like this? Is there something mentally wrong with him? I just don't understand, and I think it's the not knowing that hurts the most. Any thoughts? Please?