I've experienced that killing people in the own team becomes more and more common. Sometimes it's because they want to have the vehicle you're taking, sometimes I guess just for fun (I've even seen someone named "Teamkiller...") or as a revenge for being (possibly unintentionally) team-killed.
This is no fun at all and can turn the whole game around.
Losing a few points and having team kills in your record doesn't seem to scare many people off. For example, experienced players can easily compensate for -100 points for shooting a team mate before entering a helicopter, by then taking that helicopter and destroying tanks.
How can I remove teamkillers that are ruining a game from Battlefield Bad Company 2 Hardcore games? How can I keep them from coming back?