I want to hit my fiancee's child
Are you serious? Why hurt them?
God, the thing constantly pisses me off and in return, I'll kick him in his ass
You shouldn't hit your partner's child. You will upset the child and their parent too. Try talking to their parent and letting them know if theres an issue or tension will continue to grow.
But my fiancee is the one who has raised the kid poorly, so I'm okay with him getting hurt.
Abusing children is never ok. If you hit them you can go to jail. You should try talking with the kid to see if there is a reason they are frustrated.