I'd never kill a teammate over a weapon, vehicle, or really any other reason for that matter. I play casual for that reason. If I win, great...if not, oh well....there will always be another round. If someone gets a weapon before me, oh well, I don't hamper the team by killing them just because they aren't great with it. I just try to get it the next time it spawns. If I don't get it, I don't waste my time, nor my team's time by making a big deal over who has what weapon and how well they use it. It's called being a productive teammate and not let something like that ruin your progress.
So someone gets a power weapon and they aren't great with it? Oh well. It's called being a better teammate and doing your best for your team to make sure you can still come out on top even if other players on your team aren't doing their best. Not wasting time and making a deal over someone sucking with a particular weapon and you kill them because of it. Also, how are they suppose to improve their skills with a weapon if -Yoinks- on their own team keep betraying them for not being good with said weapon? It's it is that much of a hassle for you if a noob or under-skilled individual gets their hands on the power weapons, then go play in the Ranked or MLG playlists where people care about trivial details like that. If it's a casual playlist and you have to betray teammates for not playing up to your standards, then you shouldn't be playing casual.
That being said, I mostly just play SWAT anyway where I don't have to worry about power weapons or how good/bad someone is with them.