He went berserk from grief... (BL)

ML is an exceptionally powerful young esper who's very attached to, I'd say probably imprinted with, his guide, the MC. The MC dies during a mission however and the ML goes absolutely berserk. He doesn't accept anyone else as his guide, has outbursts of rage and for a while, couldn't control his powers and stabilize his esper energy. He's bitter, aggressive, and has become a cold, apathetic man, a total 180 from his personality back when his beloved guide was alive.

ML and MC train together as sword fighters in the academy of MC's father. One day, the academy is attacked by a cult utilizing dark powers and brutality, wiping the academy off. The ML and MC try to escape together and fight alongside, but an assassin gets to the MC and pushes him off a cliff. He survives the fall, however, the ML doesn't realize this. He thinks he's dead, so he spirals entirely, going on killing sprees and shedding blood, described as 'not human'.