The short answere is FUN.
So its fun to ruin peoples expectations once they join a server someone actually pays for, to find some guy in there that can score 999 flags in about 3 minutes, and killing everyone in the server the very same time?
If your answer is yes, I can only draw the conclusion you do it for more then just fun.
In the normal sense, people do stuff that is fun, without annoying other people, because as we all learned, annoying, is not fun.
So, basicly what you are saying is, I recognize the fact people are annoyed by me, but, I put my personal joy over those that are annoyed by my behaviour.
Which brings me to the conlusion, that besides acting selfish, its also very childish to do so.
I'm not convicting those that do cheat, this is just my thought on the case.