Like salukigirl says, you could be "practicing", being bad ahead of time so you beat others to the punch you think is in store for you; it's so much easier to do online where one truly knows the people they are "talking" to and don't have to literally face their disappointment or anger.

If you want to change this one behavior; I would pretend the people you are talking to are people you actually know; substitute thinking about your actual friends and ask yourself if you would say/do whatever you are thinking of if you were with them in person?

I'd also stop and see if you can identify any triggers that cause you to want to say/do a certain thing at the time you want to; I know I find myself in front of the refrigerator sometimes and wonder how I got there and then trace my steps back to something I just read in a book/online or am watching on TV, some train of thought that has made me anxious :-) Could be that some other person's words/what they're doing is triggering you from something in your actual life and you don't feel able to do anything about it in your actual life but here, online, it won't "hurt" anything for real.