Warning, this one's long.
TL;DR: I went above and beyond for one of my friends, stuck with him when no one else did and then he went after my girlfriend.
I had this friend, we will call him T, and we had been friends for like 10 years. In high school, we were pretty close, but in college we lived together for a year and became really close.
After we moved out T went through some really hard times. It's a long story, but the guy literally went insane for months (I'm pretty sure he was schizophrenic). During this time he pretty much lost all of his friends due to his behavior, but I hung in there and tried to help him. One night I let him sleep on my couch and he ended up stealing some shit. No one has proof, but a couple of my other friends' houses were broken into and we are pretty sure it was him. I no longer let him come over, but I still talked to him and would try and help him.
He eventually ended up in jail for a few months and when he got out I was literally the only one that would talk to him. He had done so much to so many people, that pretty much no one wanted anything to do with him. A few months pass and I find out he's hooking up with my good friend's girlfriend (his former friend) who is deployed. I'm pretty mad and I make sure he knows that, but I still couldn't drop him as a friend. His response was "well we aren't friends anymore so fuck him". I knew he didn't really have anyone else and I was always worried about what would happen if he had another crazy episode, so I hung there.
At this point we aren't hanging out much, maybe once every 2-4 weeks, but we still text quite a bit. I had started dating this girl, who we had both known for a little bit, and in our conversation I had mentioned that she is the first girl that I had ever really liked and that I thought I was in love with her. He congratulates me and tells me how she's so cool. Well, literally a day or two later I'm hanging out with my girlfriend and she tells me how T is sending her some really weird messages. She ends up showing them to me and to say I got mad is an understatement.
He's asking her for nudes, trying to get her to come over, asking if he "can lick her pussy", asking if she squirts, asking about my dick, asking if his dick is bigger and sending dick pics. I fucking lost it. I call him multiple times and he wont answer. He finally texts me a "Sup". I ask him why the fuck he's sending my girlfriend texts like that and he tells me I "read them out of context" and that he "has a girlfriend" (the same one he stole from my friend on deployment). I tell him there is absolutely no context in which messages like that are acceptable and he responds with something like "Don't be such a bitch".
I explain to him how I had always been there for him. That I had gone above and beyond as a friend (I helped him out a lot more than I mentioned above and his mom would ask me to talk to him/help her with him) and he goes after my girlfriend, the girl I had told him just days before that I thought I loved. I told him I wanted nothing to do with him. To delete my number and never contact me again. He started panicking and telling me how it was all a joke and I calmly told him to delete my number, never speak to me again, and to stay away from my girlfriend or we would have a problem. I then sent his girlfriend the messages he had sent. For weeks he'd try to get me to talk to him, but I never would and still, years later, I wont talk to him when he tries. He's had multiple people try to speak to me about it on his behalf, but when I explain the situation they usually understand and drop it.
The kid still has never apologized.