Here’s the point of all this. I know if I approach him, he will get defensive, and when he feels backed into a corner he lashes out [never at me, but I have seen him in heated arguments with other friends, my fiance, and even his parents].

This is something you need to pay VERY close attention to. You seem to be qualifying this as okay because he hasn’t done it to you, but he’s only not done it to you… yet. He will. I’ve seen this happen over and over again with friends and even myself. We will excuse behavior or think the person only acts that way around certain people but would never do that to me, until he does.  Based on everything you’ve said here, I agree with your friends. He’s not a good person to be attached to. And public personas are just as important as private ones because, at the very least, he’s not being authentic or genuine.

The way you’ve described how he was acting at the party, what your friends said, and the thing with your fiance, it seems like it’s possible this guy thinks you’re a prize to be won, which explains why he is so nice to you when he isn’t to other people, doesn’t treat you the way he seems to treat everyone else, and maybe even seems to have a ton in common (could some of that stuff be an act, too?). If this is the case and you’re a prize to be won, he will flip on you as soon as he feels he’s won you. Especially if he plants seeds of doubt in you about your fiance (which may be what he was trying to do with whatever that accusation was). He may be trying to break you two up so he can “win” you and then promptly lose interest or maybe just treat you badly for a while like he seems to treat everyone else.

Ultimately, you have to use your own judgment, try meditating on it, not thinking hard about it but see if you can feel how you truly feel intuitively about him. If something feels off, it probably is. If nothing feels off, I’d proceed with caution until he’s shown he can treat people better. Expect that how he treats others is how he will eventually treat you, too. And pay very, very, very close attention to red flags, and it seems as if there are a ton of them here. Especially if he does try to isolate you or plant seeds of doubt in you against the people you’re closest to, and be sure you’re not making excuses for behavior that should not be seen as acceptable. Unacceptable behavior is unacceptable behavior, no matter the reason.