Is it normal to do this when playing sport?
Whenever I play soccer, and someone from the opposing team is running for the ball and I'm near them. I run up and discreetly punch or elbow them forcefully in the face, stomach or groin. I also trip them sometimes as well or kick them behind the knees to make them fall.
I have very fast reflexes and I'm able to do this really quickly, and most people don't know who even did it. Other than a few people on my team who are fine with it because it helps us win.
This often helps my team win matches and we've been undefeated this season because I kicked the other team's best striker so hard that he broke his ankle and has been out for the season. No one knows it was me though.
Not only does it help my team win, but I really enjoy doing this too. I get a weird sort of warm feeling and satisfaction when I watch them lying on the ground in a lot of pain.