Yeah you're kinda ruining the point of the game and the fun for others.
It's co-op multiplayer, it's not about kill stealing or a points competition.
He's ruining my fun by running around using cheap 1 shot weapons on my targets (giving me assists instead of kills) while I have to pick away with an assault rifle and Reave. There's no skill to that so I decided to troll him a bit.
This is also why I don't usually play multiplayer in games. I'm far too competitive to the point where it's hard to have fun, though I am having a small amount of fun here. It's just like any game really, there's always going to be people using the cheap way out. If there's a bugged weapon they'll use it, bug or exploit in the level they'll use it and so on and so forth.