I think it is survivors guilt that he let's himself get treated badly. (Following is conjecture) Imagine him part of a military unit where he is friends with everyone. Then he gets send on some mission solo because he is strong and while away the unit gets killed on a different mission. Now he tries to raise the strength of others so that they do not get killed when he cannot be there to save them.
Based on what we've seen so far, he was the Summoned Hero of the kingdom.
The facts we know:
He went and destroyed the demon army, "saved" the monsters he saw were mostly harmless, and was told they were being put in a medical facility.
Later on, he would discover that the "medical" facility was really a laboratory experimenting on the monsters.
He rebels, saves the monsters, and destroys the lab.
Not sure if i rem this right. This could be before he found out the lab was experimenting on them.
(?) He then goes into hiding and sets up a classroom environment to teach the monsters about magic and life.
(?) At some point, his sanctuary is exposed and he loses contact with all of them.
(?) He now lives as an old-ass stinky E-grade (D-grade, now) teacher.
Considering his rebellion against the kingdom, he might have made a deal (?)
ie: he will stay in their sights and never gain power again, in exchange for his students to be left alone.