Well considering that you also killed a pet(the tarantula) in retaliation for his putting it on your bed(no matter that you were scared of spiders), then it's just as possible that he could press charges against you too. Just food for thought, you know?
Personally, the deliberate and cruel act of running boiling water over a living animal is far far worse than killing an animal in the heat of the moment while in a highly fearful state. One is a uncontrolled reaction(hitting a spider), while the other is a thought-out act deliberately done.
