Killing the defenders is a great thing to do. They start raging, then more of them come to get rid of you, but you keep killing them and now half their team is busy raging at you or trying to kill you and ignoring the ball. Their weaker players start avoiding you entirely leaving openings and honestly, even if there's a small chance that the guy will be on his respawn timer when the ball gets to you, it is better than the zero percent chance if you aren't killing him.
People don't like you killing them because they don't get to sit there and leech while the ball is on the other side of the field, or you are stopping them from achieving their objective.