What is Wrong with Defence??

I find it strange that some gamers out there find it offensive that someone on their side is playing defence. This was on a 16p map on an Asian server, on Fushe Pass. I played sniper with an M95 guarding the northern most flag from on top of the warehouse. A fellow team mate came up and took shots at me. What an arsehole. Man, i got pissed. I got up, switched to silencer and shot him in the head. Prick! Quit the server after that, and the TK was worth every single freaking point.