L4D2 is a very nice game: loads of zombies and weapons in coop or versus challenges, nice maps… But with every popular game around, one day or another you'll find yourself playing with assholes. This kind of players that join your game in the middle of a map, shoot everybody, kill themselves, and leave, forcing you to replay the map. Of course it's always in expert mode, the harder one, and you can't do anything because it's so fast to kill your character when playing expert. It happened to me 3 or 4 times in just 2 days, always in expert mode of course. So I designed a key binding that helps stoping "team kill", giving you some time to ban and/or kick the killer without ruining the game.
It requires a server running Source Mod, and that you have the authorization to send sm_cvar commands to this server.
Locate your autoexec.cfg file somewhere in left 4 dead 2/left4dead2/cfg/, or create one if it does not exist yet. Open it in your favorite text editor, and paste the following code:
bind "o" "sm_cvar mp_friendlyfire 1;wait;sm_cvar survivor_friendly_fire_factor_expert 0.5;wait;sm_cvar survivor_friendly_fire_factor_hard 0.3;wait;sm_cvar survivor_friendly_fire_factor_normal 0.1" bind "p" "sm_cvar mp_friendlyfire 0;wait;sm_cvar survivor_friendly_fire_factor_expert 0;wait;sm_cvar survivor_friendly_fire_factor_hard 0;wait;sm_cvar survivor_friendly_fire_factor_normal 0"
Change "o" and "p" by the letters you want to use. In this example, pressing "p" key will turn off every friendly fire damages, the "o" key will turn them back on.
When a team killer joins your game and starts shooting your teammates (and you), you might be fast enough to press the "p" key, rendering the killer mostly harmless, before he makes too much damages. Feel free to ban/kick the f*cker.
Enjoy.