Sometimes there is a fine line between discipline and abuse. While it is possible that the main character is an unreliable narrator and this character was just being disciplined, that seems very unlikely.
First is ways he was beaten. People will argue otherwise, but there is a big different between a spanking and being beaten. Here is how Wordnik describes them:
▪ a number of slaps on the buttocks delivered in rapid succession, as for punishment.
▪ the act of striking with the open hand, or with something flat: a punishment often administered to children.
▪ an act of repeated hitting or striking.
▪ punishment or chastisement by blows; a flogging.
▪ the act of striking or giving blows; punishment or chastisement by blows.
▪ the act of inflicting corporal punishment with repeated blows.
I tried to include every definition that seemed relevant and while there are some similarities, there are also some big differences.
The second thing that seems of relevance to concluding he is not an unreliable narrator is that he put up with this for a long time before rebelling. A child who is just being disciplined, whether it is with a spanking, time out, or without held treats, or anything else is less likely to rebel over one who is being beaten, isolated, or having necessities withheld. Not saying that the abused child might never rebel or that the one who isn’t being abused might not rebel, only that it’s less likely.
Given that he is most likely a reliable narrator, we are dealing with someone who is in an abusive family situation. His family are clerics. It is not stated if this is in the religious sense or the magical sense, so he could have been brought up very religious or he could have been brought up with the ability to heal people by holding his hands over them. They live in a military facility which doesn’t exactly narrow that down.
No matter what the setting, he could have gone harder into whatever his family was doing trying to prove that he was better at it then them or he could have chosen to walk away from it all. Maybe he truly believed in their religion and it sickened him to see his family perverting it or he thought it was sickening to see his family who were supposed to be healers harming people. Or maybe seeing them in action convinced him that there was no god and that humans are just horrible. If his family was the healer type of clerics, it’s also possible that they used this power to hurt people. I’ve seen stories were healing the undead can kill them and others were reversing a heal or making a wound worse via an anti-heal type of skill are possible.
What kind of clerics do you think his family were? And do you think he was the only one being abused? Maybe after he left he made it his job to right their wrongs.