My God. Someone is taking things a little too seriously. Bad roommates are not fun, but let's not talk about guns here (Live in Arlington, TX). I have three roommates, only one of which I picked, and sometimes they do things that upset me. But, they are just roommates, and they will be gone at the end of the summer, just like your guy.
It sucks when someone disrespects you and takes advantage of your stuff. But you have to realize a few things. When you're a freshman, you've probably never lived out of your parents home. At your parents home (if you remember) everything is free to take. Parents don't care if you take anything you want from the fridge or pantry, use fifteen kleenex boxes an hour, but they do get upset if you have friends over till 8 in the morning.
As a freshman, you are, for the first time, on your own without anyone telling you what to do. So, when you tell him what to do (that's how he sees it), he thinks you are trying to replace his parents, who he probably still resents for being controlling. Your first year or so in college, for most people, is a rush to be free, and having people over at all times and making a fool out of yourself is not only usual, it probably is a good thing. Gets it out of your system and shows you that responsibility can be a good thing.
I know this doesn't help you very much and you're probably thinking "who the hell is this guy?" I'm just someone who was somewhat like your roommate and now could be like you with my freshman roommates who don't respect some of my things, but I take a laidback (passive like someone said) attitude and was willing to realize that, in the grand scheme of things, $20 isn't that big of a deal, and problems can be solved without writing a speech or calling the authorities.
I'm not sure what the perfect solution is to your problem, but it would definetly involve taking a step back and looking at what is going on and how your behavior is escalating beyond what is associated with being just upset. It's hard to talk to people who initially don't respect you, but I promise if you catch him in the right mood, he'll talk to you and maybe you can get some concessions from him (late night guests only on weekends or something. It's all about compromise. He wants freedom no matter what and you want silence no matter what, but that's just not going to happen this time. Try your best to find some common ground, forget about the threatening letters (freshman can be dumb most of the time) and work it out. It can happen.
If all else fails, it will only suck for another month.
Maybe I'm a pacifist, but I think looking back a few years from now you will say to yourself "why did I get worked up over that so much?"
Just my humble opinion, feel free to ignore it.