Team killing is well annoying, considered a grief and easy enough to do. It can only be done by a soldier, and a demoman without cheating. Its easier to pull off as a demoman using his pipe bombs than a soldier with his rockets. Criticals would have to be on for starters as itd be hard to kill someone unless they had low health and were standing still. First find someone who is stationary such as a engineer or sniper because the pipe bombs have a delay as a demoman. When you get a critical bomb hurry and hit start brining you to the game menu and hit change teams and click on spectate. The game will consider you on a enemy team but you cant spawn and the bomb will criticaly kill your team member. Afterwards switch back to your team and be hated by your team. Easy enough to pull off but you have to be fast enough to do it you have about a 3 second delay in the bombs.