Most people just ran around team killing, and I suggest this is because they didn't realize it's a team game at all.
Some suggestions:
* At the beginning of the screen show a message that tells the player 'You are on team RED'
* Add a message for players who are team-killing - like a huge 'TEAMKILL' popping up on their screen.
* Also add a penalty to prevent people from team-killing (like a 10 sec respawn delay)