It sounds like he hasn't had any "consequences" to his behaviour - obviously not at home or at school (um...I would think any of those actions involving hurting other students are causes for expulsion).
If you have no say in whether he stays - I say work on the little bad behaviours first, taking away "priviledges" in class and then work up to working detentions - scrubbing grafitti, etc. Setting boundaries of right and wrong, cause and effect might stall the little future serial killer.
Good luck!!!!!!!