It's tragic for all involved. However, this kid was as big as a large male, and had obviously been out of control... dangerously so... for years. I don't want to heap blame on anyone here, but if police are putting their lives on the line dozens of times over a few years, clearly this boy needs to be either medicated or placed in a controlled environment where his needs can be safely met.
Perhaps everything had been tried, medication and needs-specific education, etc., and had all failed. If so, the parents must have been at their wit's end. Still, I can't fault the officers for what they did, given his previous history of attacking police with knives and the fact that he had just sliced the guy's partner up in front of his eyes.