That he went through an even worse training than he puts his students through explains much. He is so broken that he doesn't realize he is breaking others.
I hope we get a flashback of what happened. The guy started as a jerk but made enough progress to deserve more than an off camera death.
Thanks for the big batch of chapters.
Hopefully our main has learned that a life or death battle is not a great time to test unknown skills and that the skill he tested anyway would be best used in bursts for getting distance enough to use his usual kill method.
Conquering labyrinths to make money? The girl could buy her own crepe factory. I wonder what the calorie balance is for fighting monsters all day but eating as many pastries as you can...