Though I share your frustrations, the key might be in the "hasassing" nature of his actions. There seems to be two organizations capable of helping the targeted victim: School and police.
Evidently, he was expelled from the school that he attended with the targeted victim. Expulsion from school, however, is an administrative process and is relatively easy.
That leaves the police. Police need actual criminal behavior to take action. At some point, harassment (non criminal) turns into stalking which is criminal. That point, however, may, or may not of been reached yet.
It also looks like the police were responsive to the harassment as the murderer complains to a friend that the police stopped him after pulling a stunt. But.... there might be limitations on how much attention the police can give one individual verse others being harassed or stalked.
I may lean too much towards being protective of the police. They might have been able to do more. Then again, they may not of. But, at the end of the day, US police operate in a democracy and need clear violations of written laws to take action. Likewise, they do not have the proverbial "Crystal balls" to determine which harassing ex stays administratively creepy and which will eventually commit murder.
