Fresh out of sympathy for myself

Ok story time. I asked a girl out, we hit it off, but she's really busy and doesn't have time for a relationship so she says she wants to be friends. At first im super depressed, i guess because i got my hopes up, but still its ridiculous to cry over something this trivial. But eventually i come around to the idea and we hang out a bit as friends. Then i start to convince myself that she's stringing me along for the attention (shes in rotc which means she has a lot of guy friends but i started to believe she was doing the same thing to them and it was all some sort of twisted game to her). So i talk to her about it and she understandably gets pissed, says she doesn't want to be friends anymore and isn't interested in being scrutinized for having friends or a life, and then deletes my number. A couple days later i wake up at dawn to the rotc kids doing their chanting and marching and i just get so unbelievably pissed for no reason. Then i text her this (cause i still have her number at this point, god knows why):

Me: Youre planning on killing people for a living right? Does that bother you at all or nah?

Her: Im sorry? Who the hell is this?

Me: Enjoy ptsd you sick fuck

Her: \[my name\] if this is you then this is really messed up

Me (trying to make it look like im someone else): what is that your bf? Sounds like you two need to work some stuff out

She doesn't respond after that and i delete her number. Ive always thought of people who behave like this as the scum of the earth, and now i realize i am literally one of those people and its only a matter of time before i do something that is seriously beyond fucked up. I normally don't want to hurt anyone, i just get so fucking mad sometimes and say whatever it takes to hurt the other person. I used to fantasize about burning down my ex's house. This isn't just mental illness, this is a headline waiting to happen, and all i want to do is murder the sick bastard who is living in my skin. Obviously that is the only option, right??? SURELY i would be doing the world a favor. I have tried so many times to be better, to change, and i always just do some shit like this. I constantly hurt everyone who loves me. I am out of self pity, i am out of sympathy, i am done. But i am afraid to die so like the coward i am, i decided to let some strangers on Reddit talk me out of k\*\*\*ing myself. Unless i really am right, and you all agree, and that would make things much easier and simpler.

Im. So. Tired. I can't do this shit anymore. I just want to scream and pound the walls until my voice gives out and my hands bleed. Im positive i need to be locked up in an institution or something, given enough pills to make sure i never hurt anyone else.