"Wait until I hero, then I'll kill you." It just screams game mechanics to me and takes me out of the setting a little. They may as well be saying, "I could beat you up if you weren't tougher than me."

It's a sentiment that should be born out of frustration, not arrogance. They don't have to pay an enemy hero respect to get the message across. I'd infinitely prefer "One day I'll kill you, I swear it!"