His teacher is responsible for this, he made the plan by cooking up a forbidden spell (he needs to feel a lot of negative emotions).
MC is chosen as a sacrifice for the spell and his "GF" is in for it, she's a bitch and golddigger.
The spells is released and there's a timeskip of two years, they think he's dead but he's back for revenge.
Basically he kills the hero, turns his "GF" into a monster and trap his teacher in ice forever and builds a harem with girls that were formely his teacher guinea pigs