I was threatened like this when I was a child, because I was the boss of the family. I once had a problem with my little friend. She said, I told my brother to take care of you. I was scared at the time, and nothing happened afterwards. I always remember it because it left a deep impression on me.

The reason why the boy behind you said such things to you may be because he likes that girl, threatens you and makes you flinch. If you like that girl, don't be afraid of his threats. He won't do anything to you as before. He just pays lip service to show his strength. You just need to be careful.