Ignorant n00bs shout this when killed in such a situation, when they should have been more worried about finding a safe location to type, or actually playing, but they bitch at someone who might have snuck up on them from behind (so the typekillee might have died anyway even if "alert"), or shot first and asked questions later.
Most likely, saying "Typekiller" was made popular by Quake 3, which featured an unmistakable blue box with a ;) over the head of someone that was currently typing. In this way, there could be no excuse of "I didn't know you were typing"
This all has something to do with "honorable combat," which is an oxymoron unless you still live in the Middle Ages. The Code of Chivalry was kind of forgotten when knights were replaced by M40A1's and SAW's.
Player: WTH?! Typekiller!
Attacker:*Takes adavantage of victim's inability to protect themself and kills them*
Victim: Dude, stop type killing, ya Lamer.