The cop and the dead guy had previously worked security at the same place for many years. They knew each other. My guess is they had some prior issue, and now they don't.
The cop is toast. He knew he was on camera(s), yet he persisted for nine minutes. I can guess a few reasons why, but we'll probably never hear which one is correct.
That would explain why the cop was vicious to the guy, but would not suggest that the DS planned this in advance, as opposed to taking advantage of it when it happened.