Moonbear,Remember, as we've been hearing, Cho was also taken into court-ordered custody (at that stage, not really different from an arrest) for his creative writing papers...did we ever hear about that in the news at the time? No, it was just something administrative that is done when someone that disturbed is identified as a potential threat to self or others, and nobody would have thought to make a news story of it. Perhaps that his illness was not taken seriously enough allowed him to remain out in society and to do something so horrid. Here's another kids who seems to be showing similar disturbing, violent tendencies, finds humor in it, and makes threats against the school and teacher, and you want the teacher to not notify the police to do something about it?
No, maybe he won't be the next school shooter, but someone who finds humor in shooting people and having sex with the corpses might turn out to be the next serial killer. Who knows, if there's an unsolved murder in the area that fits that description, the police probably wanted to detain him long enough to make sure it wasn't a confession.
Cho was detained for a psych eval on the basis of stalking--had nothing to do with his writings, so while I'm not necessarily disagreeing, I have seen examples of this where at a high school, kids gets detained for 3 days in a psych hospital on the basis of a graphic drawing. They (teachers, et al) were being careful, but an involuntary psychiatric hospitalization seemed overboard IMO.