It's the same basic thing people have been complaining about for years. People think video games promote violence and like to cite columbine but then forget about how we had rampage killings for literally centuries if not millennia before.

Playing loads of COD will definitely condition you to be better at the game. Meaning you will instinctively respond by clicking the mouse when you have a target. Probably will improve your target leading skills slightly, but increase violent impulses? Hardly. Spill over into real life? Firing guns is a lot different than playing a video game.

It's like saying, playing racing games will encourage dangerous driving habits and speeding. Or that playing the Sims will encourage controlling and psychopathic behavior and give people a god complex.