Mine aren't really as bad as some of these peoples, but I've had 2 terrible roommates and 1 kinda bad one.
Freshman year I had one of my roommates switch rooms with another kid (the just bad one, not terrible) because he wanted to live with his friend/sports teammate. Well the new kid started off fine, but he started doing weird stuff or just straight breaking our room rules. One thing is he would wear this cologne, our other roommate (good friend of mine) would comment to me that it smells really familiar but he's never seen the kid have a bottle of cologne or put it on. Found out pretty quickly he was actually just using my buddies special cologne he wore when taking his girlfriend out/going to her families. He would eat our food, but not in front of us. He would ask whose it was, then wait till they left and eat it. The worst was when he broke Rule #1. He went and turned off a PLAYOFF NHL GAME (the local team we all love) while we were watching it because he didn't like hockey.
Terrible Roommate #2 was from my junior year. This kid was clearly some kind of mentally disabled, but whatever not like I haven't had dealing with mentally disabled people before. This kid was 100x worse than anyone I've ever been around (and I have a cousin in her 50s that is mentally like 10). He would use ALL my bathroom products (I'm talking toothbrush, wash cloth, etc. If it was in there he used it), and that's only when he would actually use such things. The kid wouldn't shower unless forced. He would pound on my door at random hours for inane reasons (yell at me for eating his food that I didn't eat, yell at me for not having facebook, you get the idea.) Now this kid wore diapers sometimes, which in itself is no problem. I'm not gonna judge since there are legit reasons anyone would have to wear one. But anyone that has ever been around kids in diapers you know how to dispose of them, well this kid just threw them in the trash without securing them. He was alone at the dorm the first time he did this and we came back to the entire dorm smelling like a bunch of cows took a literal shit on everything. He was just in general really unhygienic, I had to clean up after him all year. It gets worse but I'll move on to the 3rd one.
This kid was my roommate with Terrible #2. He was fine at first, a cool dude, a smoking buddy, just really chill. It was towards the end of the year when he went off the rocker. He was on pills for a mental disorder (like schizophrenia and the like) but decided to stop taking them because he was dating this girl. He would threaten her and make her feel uncomfortable so she broke up with him so he went off the DEEP end. He started freaking out all the time, spreading lies about me and her because we were friends and spend a lot of time studying together (we had 3 classes together and where studying with like 4 more people every time). Before they broke up, but when me and her started becoming chummy I got arrested and (while we don't have direct evidence) we think he snitched on me and tried to set me up a second time.
I know live on my own and refuse to ever have a roommate that I'm not in a relationship with.