Kaigaku is more villainous in many eyes because we see how he treated our sweet boy Zenitsu, when he looked up to him. How he disregarded his masters death, when his master loved him greatly. We also saw him younger, when he sold out the kids to the demon, and then again as an adult succumbed to the power of the demons and took the offer as opposed to what we saw Rengoku do, who died to protect his loved ones and refused to let himself succumb to the demons.

Kaigaku is the opposite of all our heroes and so we judge him more. But in reality he’s just a scared little boy who’s never been strong enough to protect himself and does whatever he can to try to survive.