
I watch the anime first. It was a good anime but from my experience with anime adapted from novel, a lot of stuff got skip only focusing on major plot because episodes are limited and not enough to cram everything in.
Sometimes those details that was skipped are the one that give life to the story and without it the story still advance but dull.

So because she doesn't fit in, they disfigured her? and then all of the bullies went ahead and live a normal live as if they never hurt a fly while the poor girl was traumatized and sent to mental hospital. Whelp, they deserve every torture she did to them.
I'm thinking this is also why she kill all the unrelated party that comes her way. The teacher, police, doctors, nurses etc. It was obvious that what happen to her was no accident, someone plan it out but nobody was caught. I'm thinking the case was never brought to light and put under the carpet all this time. So she's thinking nobody is on her side. Everybody are her enemy. She only actively hunt down the bullies but if other people got involve then they definitely belong to the bullies side and therefore needed to be kill.
In the novel, he didn't leave the hero party because he want to live a carefree but because he was kick out. He still want to fight the demon just not at the front line. However, its been quite a while since last read the novel. I might be wrong.
It says in the alternative manga title that he was kicked out
The title did say that but from what I read in chapter 1, there was no mention of him being kick out. The chapter implied that he retire voluntarily.
He assumed they didn’t want him based on one guy. He’s an idiot. If it’s not done in front of the team it doesn’t count.
The manga kind of simplified the whole situation and made it seem like Ares was the single reason why he left. In the LN, it's implied and mentioned that the other team members also didn't seem to appreciate him much and looked down on him a little. Though they didn't want him gone, unlike Ares. But yeah, the MC kind of lacks self-esteem, hence why Ares's comment about him being unneeded was the final straw for him.