UPDATE: Issue resolved. We kicked him out last night. And as Jaywa stated, I do indeed believe he's psychotic. Here's how it went down (sorry for the long read, but I figured after all the great advice, I'd provide all the details):
Things went along as usual for a couple of months. He kept being himself (complaining, tossing out know-it-all comments all the time, etc), but we dealt with it. Then in October, more drama. Our rhythm guitarist was asked to move his car for another band to load in. You could see he was really angry (not sure why - probably just having a bad day). He accidentally dropped his wireless relay, tossed out a couple expletives, then walked outside. When he comes back in, our drummer, in his typical know-it-all style, says "that's why I don't park there. I've never had to move my car before." Our guitarist says "yeah, I fogot, you know everything." Our drummer proceeds to pack up his gear and leave in the middle of practice! Guitarist calls him out, asking what kind of p**** would act like that, but he just walks out without saying a word. But here's the best part - that night he reaches out to me - yes, me - the guy he called every name in the book and insulted on every level just 2 months prior - for sympathy and confirmation that he did the right thing! Ridiculous. I didn't respond.
At our next show things went rather smoothly. Afterwards we packed up our gear, and the guys (and a couple of our GF's) were all outside talking. We look over and he's pulling out of the parking lot. Didn't say a word, just packed up his stuff and left. So we had a group discussion about how it's come time to find a replacement, but we'll wait until we get through the end of the year.
Fast forward to Saturday night, New Years Eve. I broke my toe that afternoon and our rhythm guitarist was REALLY sick (flu-like symptoms), so we were already in rough shape. As we were packing our gear to head to the venue, we got an email from him (not a call, an email

) saying he had another kidney stone, doesn't feel he can play "effectively" and "really wants to get out of the gig". It was another typical ridiculous "woe is me" email, and he dropped the N word again (said he didn't want to feel as though he was "n*****ing out on us"). I don't even know what that means. Our vocalist calls him and he answers with a weak, painful voice. Tells him he thinks he's going to have to go to the ER. So we start calling everyone we know looking for a last minute replacement. Then we had to find out the BPM's for a couple songs, so I called him. He sounded perfectly fine, and his tone made me feel as though I were annoying him because he was busy with something. Really p*ssed me off. So we all agreed he was done. We ended up playing to an unprogrammed drum machine. We had fun with it, and the crowd and venue still loved us, but because of his cancellation we decided to apologize to the venue and forfeit our pay.
So last night we all get to practice. He walks in, stands at the door and won't make eye contact with anyone. Our vocalist says the negativity and drama is affecting everyone, and it needs to stop. He goes on about how he's never attacked any of us personally, but we always take things personal. Says he's done nothing but show us all the utmost respect and try to be our friend, but we clearly aren't interested. Then he attacks our rhythm guitarist saying he's being "sabotaged" by him. Our lead guitarist (who never says ANYTHING controversial) had heard enough and said "I'm tired of the drama. You're done." Since it didn't come from myself or our rhythm guitarist, he had nobody to attack, so he slammed the door and left. We texted him saying we're putting another lock on the door and we'll meet him there when he wants to get his drum kit, and he goes on about being insulted that we think he'd steal from us, and he'll bring the police with him and get his stuff when he wants. (which he can't do as he's not on the lease)
So, although we were left hanging on NYE and will be playing our show on Friday with a drum machine, we finally dropped the hammer and have some good prospects we'll be auditioning in the coming weeks.