Cop on Cop violence

It is reported in Washington Post how a Puerto Rican cop took three of his fellow police officers hostage and then fatally shoots them. He was apprehended and is now in hospital.

How do you qualify or vet a police officer  that he ends up killing his own people? A lot of questions, not  a simple story.

https://www.washingtonpost.com/news/post-nation/wp/2015/12/28/puerto-rico-police-officer-shoots-and-kills-three-fellow-officers/