I think - based on the previous convictions that he was less a thug and more a mentally ill - a seriously mentally ill person.
He should not have been on the subway system - he should have been in a mental asylum.
SCOTUS ruled that crazy people can’t be locked up unless they are a threat to themselves or others. Neely died happy, knowing that he was free to terrorize a subway car full of commuters, thanks to the Baker case.