TK is fun if the game has gotten boring, and the person you're killing is up for it. The level of absurdity involved in it makes it a blast.
Some games encourage TK, i think, by making it so damn funny. Its pretty funny to see someone jump infront of a plan that is committed to take off in BF2. Its also kinda funny to just be there talking to someone while there is a lull in the action, and then just randomly pull a pistol out and blast his brains out. But it does get old, and its certainly not funny for the person getting kill, if they aren't in on it.
If you're gonna TK, make sure your target is cool with it. Otherwise, save everyone the trouble and just ban yourself from the server.