I dig your name~! Is it in reference to Colby's Black Mamba title?
, many people tend to be trolls because they have self esteem issues and sadly, you're their chosen target to take it out on. I've started a few roleplay threads in which someone else wished they were the main character and butt in, or etc. They might be jealous mad at your skills, or wished to be the character in their instead, and assume an identity they are not. For instance, I've had this girl actually assume the identity that she was one of the main characters, and think that she is the character- some people need to pull their butts back down to reality. I guess she was jealous that I roleplayed the same character she does. People don't realize that anime characters don't belong to anyone and that we are all allowed to roleplay them. So in your case, some guy may have wished to be your protagonist, and had an issue with your fanfiction?
I don't know, you'll meet jerks on every forum you'd go to. But there are still a few good people left around.