If we treated criminals and crazy people like criminals and crazy people this probably wouldn't have happened at all. We used to separate criminals and crazy people from normal society in order to keep everyone safe.
Keeping them separate protects average everyday people safe from them, and it keeps them safe from average everyday people as well. If he was seperated from society then he wouldn't have been killed.
When you're dealing with a country with 300 million people in it you have to do everything that's in the best interest of the majority, which often times mean having to do so at the sacrifice of the few. And the best interest of the majority is to treat criminals, freakazoids, nutballs, and dopeheads as what they are for the good of the whole.
Our system is to blame for his death because it let that guy run around in public. The system failed to protect him by separating him.