I think it is very unrealistic to expect impersonal institutions to be more conscientious and protective than his family, neighbors, and friends. In fact, his crimes are so exceptional that noone who knew him expected them. Why would institutions do better?
He no longer lived with his family, and it’s apparent that they had no idea of the severity of his condition, quite likely because it had progressed since the time he moved away to go to college. He probably went home for school breaks, but when he just didn’t talk (which had been his pattern since early childhood), and the parents had no access to the disturbing writings he’d been turning in at school, or to the record of complaints against him at school, or to the info that he’d been declared mentally incompetent by a court at one point, what could they really do. He had no friends, and didn’t speak to his “neighbors” in his dorm (or even to his roommate), or to his parents’ neighbors, and none of these people (including his parents, once he’d turned 18) had any authority to do anything for him without his agreement, which obviously wasn’t forthcoming. Courts are the only institutions in our country that have legal authority to make an adult do something s/he doesn’t want to do, and that’s how it should be. Cho’s family appears to have been the sort of people who would act in his best interests to the best of their knowledge and ability, but that’s hardly true of all families, and that’s why a court needs to make the decision. In many cases, a court appoints a family member as legal guardian, if a suitable and willing family member is available.