Anger issues?
Lack of calcium and vitamin d?
Scared of spiders?
He probably scared of the spider, so he kicked right on the spider as much power as he can, while at the same time hitting the leg against a solid object (wall, floor, etc) and broken his bones.
First of all I think he's stupid... why killing a spider? It's not harmful at all, as well spider helps to keep pest (like mosquitos) away. Second if he has to kill the spider, he could of just take his shoe (or something) and smack it.