I've been really upset with this actor and I told him I'm going to get someone to break his leg.
Why would you be so upset that you would tell the actor you're going to break their leg?
I just want to break his leg.
Threatening someone can be seen as a punishable offense. The person who is threatened may report you or even do something to stop you from hurting them first.
I hate his acting. The leg is only for a stunt.
This is not can be a good stunt for you to pull off. Like I said you most likely wind up in legal trouble. Take a step back and think about this before you do something you'll regret.