Forgiveness of old friends in need... Where to draw the line?

What does one do when they're torn between forgiving and helping someone who needs it and receiving nothing but ** in return, and just watching them flounder?

I know it's a questionable idea, but I'm considering trying to make nice with a former friend, because of how awful I feel for him. The guy's a trainwreck. He objectifies the people around him and treats them as means to an end, has an explosive temper, and is completely unrepentant about his crummy attitude - and he's totally alone for it.

I've often considered trying to make up just so that he'll have someone around that he can have fun with, but we have a history. About a year ago we started going out, and a few months after that I discovered that he didn't actually like me, but out of desperation he jumped on the first girl who'd ever given him a chance in the hopes that he could finally get laid. Needless to say, things came to a screeching halt. He's like this with women as a whole, and is frequently resentful towards us - and was angry with me for months when I eventually came out as a lesbian two months ago because he saw me as a "man-hater" and as competition.

There was a point where he was trying to make up to me, but a little digging and he admitted that he was just trying to keep the one female friend he had. I honestly don't think he feels an inkling of guilt for using me, and he's only upset about what this has all meant for him. I'd be in a one-sided friendship if we started talking again, and that stings.

Every time I think of forgiving him, I find a hundred reasons to clash with the one reason I keep thinking I should: because not only can he not get a girl, he has no actual friends. He's in my social circle (I use the term loosely), and I have to watch the madness on a regular basis. It makes me internally wince. He might have treated me horribly, but I still feel for his problems.

There's a line, of that I'm sure. I just don't know where to draw it anymore. Is it ever worth it to let yourself get pushed around a little if you think it's going to make a difference for someone else?