Invasion Etiquette?

Now, I am a SL 66 Darkwraith, pissing away some time in Anor Londo invading people. I ususally find that it is tactically advantageous for me to fight in the chamber just outside of Ornstein and Smough. But over the course of this night I have received a few messages saying that I am a scrub, asshole, etc. I just responded sayingt that this is Dark Souls and that I am a Darkwraith, but that going tiring after the 2nd message. So, I was wondering if there is some sort of unwritten rule about this sort of thing.

TL;DR I invade people just outside of O&S room and people do not like it. Am i doin it rong?