He did also teach the monster girls magic and life lessons earlier in life after presumably rescuing them from being experimented on (and flashbacks show he was the one that gave them to the experimenters, another thing he probably feels intense guilt about). Maybe once he "put them on the right...
Looks to me like Washio had a hand in killing the monster girls parents. Notice the dead corpse of the larger spider in the flashback panel? I think I see a large tentacle underneath rubble on page 14 as well. Probably a contributing factor to his self-deprecation and wanting to push them away...
I kinda hope that the elf teacher actually does take over as their supervisor at some point and is shown to be woefully and completely inept at handling them as he can't get their respect at all.