Yeah, this is the possible exception on the character development front...
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I sort of figured John's instability and violence were chalked up to his disastrous experience at his previous school. But we got the flashbacks, and no, it looks like he had the same pattern there: nice kid before he starts using his powers, over-the-top violent monster when he does. This feels like a mistake on the author's front: if resolving what happened last time is necessary to fix this, how does that fit with him having gone down an awfully similar path without the prior experience?