Most everyone that replied to this post is right..........You were probably lagging, especially against this guy more than the other players on his team.
Killcams arent accurate as stated by others, especially when using the wii-mote.
An example of what happened to you, happened to me last week. I was playing at HIJACKED and I ran into one of the rooms, looked real quick and no one was around and then I ran upstairs and around the corner. All of a sudden I heard BOOM and I was dead. I looked at my girlfriend and said what in the hell was that? The kill cam showed a guy shooting at a wall that I wasnt even near because I was up the stairs around the corner and he couldnt even see me. It even showed his bullets hitting the wall in the opposite direction where I was. She said, that aint right, I said, I know, Im lagging. I said, on his tv screen I probably was in front of him but on my screen I was around the corner upstairs.
I leave when Im lagging that bad because that one guy will kill me easily the whole game because of lag.
To my knowledge, I havent seen a cheater yet, Just lag making people get what appears to be mpossible kills.