Every now and then I remember a time playing volleyball back in high school. There was a specific game where I could tell one opponent just could not handle getting served to, and I sent it right to the guy maybe 4 times before stopping (possibly because this question kept bugging me).
It becomes immoral when we use hurtful tactics, such as deliberate physical violence with intent to injure, or go against the agreed-upon rules of fair play. When we take it upon ourselves to persecute or bully, verbally or physically, we are going beyond the morals set out by good sportsmanship.
Finding the weakest link is actually moral in terms of your own team, which I presume wants to win the game. So to have ignored your advantages, would not have been good team work.