I usually think 'son of a *****' and then get on with the game. Raging about how someone got lucky and randomly killed you is incredibly sad and pathetic and I try to have no part in it.
Personally though, something like 15 kills and 0 deaths is a rarity for me as I'm one of those assholes that uses things like Lightweight, Marathon, Commando and Tac Knife on Modern Warfare 2 online, all in the hopes that some little 12 year old with a mic will start raging at me. Getting a high kill count is easy enough, but getting a low amount of deaths when half the people I come across start carrying shotguns to deal with me results in me getting a lot of deaths.
I'm also that guy that likes to take a riot shield, RPG and claymore/C4 with Scavenger and Danger Close.