While doing a light reading (LN) to answer you. I stumble upon this.
Guy tricks and house arrest(kidnap) alfine by using a fake MC Corpse and then proceeds to ruin everything by having a supervillain plan exposition(very detailed exposition) with his partner(who is supposed to already know the plan). Alfine overheard because she is still free to wander the guy's residence
I just had to spoil that specific part because it was baffling.
To answer your question.
It seem that the early part is basically a bog standard OP MC with a childhood friend revenge story just tacked on(as it's possible to remove this aspect of the story and little would change).
(Childhood friend revenge story are apparently getting/ got popular on syoestu. So mabye abusive childhood gf is just author copying the appeal of other popular stories)
