I'm mad at my classmate for making empty promises and screwing me over because of it.
What was the empty promise that your classmate made?
He was supposed to help me with an assignment but then he bailed.
I can understand why you're upset with your classmate, because if he promised to help you with an assignment then he should stick to his promise. It's extremely rude to promise something and then not deliver, because it can really put the other person in a tough situation.
I'm glad you are in my corner. I will kill that little piece of shit once I get the chance!
It's totally reasonable to be mad that he didn't follow through on his promise. It's not worth reacting with violence though. That will possibly get you into more trouble and you don't need that.