I don't have a problem with it.
There has been plenty of games over the years in which you kill innocent bystanders (people have already mentioned GTA). Also, from the footage I've seen, it doesn't look like Infinity Ward glorify the actions at all.
It's so neutral and "disturbing" that I think it does well at setting the motivation to oppose the terrorists for the rest of the game (like the first MW game did with the opening sequence).
Perhaps setting it at an airport was in bad taste though, given the state of the world at the moment (I personally don't have a problem with it, but if there's anything "wrong" with the footage, that'd probably be it), because there aren't many games (any that I can recall) that puts you in the shoes of the murdering bad guy in a airport (which obviously is significant in this day and age).
Honestly I don't have an issue with it, there's been much more disturbing images in films for a while now and I think it's just another attempt by the overly conservative to grasp at one of the few areas that they have maintained control over recently (because clearly they don't have much control over film and TV now).







