Amargith speaks the truth. I will admit to getting like that sometimes. I'm getting better as I get older though. When I was younger esp. I would lash out A LOT, because I've been hurt a lot. It's like "f**k it, I'm sick of getting hurt, being nice doesn't get you anywhere" and you lash out at people becuase you expect them to hurt you. That's my experience anyways.
Edit: Also, I've been told by a few people that when I am nasty (more so when I was a teenager!), I find the right thing to say that will really hurt someone, like I just know that one thing. And I gotta admit that's true. :| This post makes me sound horrible - it was more like that when I was going through my angsty teenage years (I am only 21 now mind)
Also, I have been abused myself.