I remember that it was his ex teacher and the organization behind her that stopped Zeff though? They were the ones who thought it was a good idea to trap someone in their own mind for thousands of years without killing them. Like what do you expect to happen when you do that? Of course the guy would go crazy. And with all that time to think, of course he would think of ways to get revenge.
Vol 4. Chapter 32 has a bit too much dialogue for the author clearly ramping up Zeff's emotions. They made him have a flashback and a inner monologue to confront his past, yet he's posing like some hero taking down a demon lord instead of just slashing him. Furthermore, even after the association mage's arrival, can't remember her name since she's literally just plot fodder, he could have argued for an actual punishment since he was more disgusted towards Grain's personality/actions than the fact he was a mage. Too concentrated on the fact that Grain's a mage when it's his attitude that makes him horrible. Kinda just flew over their heads that even if you take away his magic, he's still gonna be an asshole.
Granted, Ein's evolution was not accounted for, but at the same time Zeff should have been able to move forward quickly.
One thing we both agree on is that the writing is terrible. Protagonist is setup to be someone with past knowledge and has clearly shown how he can act on such knowledge, yet his actions dictate that he is devolving mentally outside of the fact he's kinda oblivious to his harem's advances because he sees them as too young or some crap.
The story has been lackluster, harem is incredibly dumb here, and the protagonist is some wishy-washy-esque man chasing after his dream of red magic when he doesn't have the talent for it. Due to this and his meaningless behavior, he's facing a villain he created himself because he didn't have the heart to kill his enemies.
Literally what in the heck are we as readers are supposed to take away from this series? This may as well just be pure ecchi since nothing else seems to matter.