People criticize this manga for it's unclear plotline and confusing characters, which makes sense. Neither of them are as simple and linear as people want them to be. Most don't look behind what they percieve as confusing or irritating. They see the sensei as a quote "shit head" and think that he is supposed to be held captive as for how bad of a human being he is.
This is quite ironic considiring this way of thinking is the problem itself this masterpiece tries to adress.
It needs to be understood that this piece of art is not just simply a soft romance manga, but instead criticism about the values in our society.
As for sensei is not just a character in the manga but instead the embodiement of the more "evil" or "weak" side of humanity, as is seen in his short temper, impulsive resort to violence, hense he was ultimately excluded/excluded himself from society.
While others turn their back to him yamada does not. Without any ultirior motives (as shown by the fact that she doesn't see him as a love interest) she comforts him, with the sole reason of him being "hurt". Yamada is the "good" side of humanity as she keeps him company and tries to heal his wounds all while not gaining anything herself, which is what many find confusing.
Truth is hurt people hurt people, no one choses to become a "bad person". Locking these humans in a cold cell with iron bars, suprisingly enough doesn't reform them.
Love and affection instead can help humans recover as for humans are nothing but a mirror of their surroundings.
Which is why sensei through yamadas affection was able to improve and not by being locked up or having to pay a fee, only further keeping him away from society.
Should someone actually read this comment, i hope you give the manga a second try with my interpretation in mind and let it assist in forming your own opinion.
(english is not my first language, sry for the typos/ bad gramma)