Typically in the call of duty series, most of the game modes allow you to see the person who killed you and how you died immediately after death, this is called the 'killcam'.
A 'killcam cunt' is someone who is so bad and dies so often, the only way they can find anyone to kill is from watching the killcam and proceeding to try and kill that person who killed them. Killcam cunts will repeatedly rush the person that is killing them until they finally do kill that person.
Moment later that killcam cunt will die to someone else and repeat this process until the match ends.
"FUCK! I was 1 kill away from that killstreak, but that killcam cunt just had to fucking kill me."
"Just fucking WOW! Of course some killcam cunt kills me when I'm camping with a silencer, cold blooded pro, and ninja pro on this class."