well, the particular manifestation of his psychosis isn’t common (eating people), but psychotic disorders aren’t super uncommon and they still aren’t as well understood as we might like. unfortunately, not everyone responds well to treatment - it can take some effort to figure out which meds are effective and for some minority of patients, nothing really helps at all.
in this case, it sounds like he responded very well and perhaps (haven’t read more than what’s posted above) had not been treated before. at the very least, treated insufficiently well, clearly. it doesn’t hurt that he was forced to receive treatment and supervision for a long period of time, and given his high profile case, maybe got the best psychiatric care available to him. i’m happy he was able to get some help.