The shinigami are an example of cultural entitlement and peer pressure that creates a world view that sets themselves up as the dutiful shepherds of all existence. They operate under a cloak of self-righteousness, backed by iron-fisted dogma that is further reinforced by the government they serve. Because they separate themselves from humanity so, by living in their beautiful city in its pretty white walls as opposed to the slums where the "humans" live, they also separate themselves from such ideals as fair trials, evidence and innocent till proven guilty. They are the guardians of the universe, of COURSE they know better! They're shinigami.