Let's say you only play dark souls for the PvE. You haven't even heard of duelling, or if you have you have no interest in it. Your only experience of PvP is going to be people invading your game and *** your **** up, ruining your enjoyment of progressing through the game.
Maybe this explains why SOME people gank; they see it as revenge against people who they see as threats to their enjoyment of dark souls PvE.