You mention the fact that they had worked together. That piece of the puzzle was on my mind initially when the story first broke. It occurred to me that something might have happened between them that triggered Chauvin's actions. Was it a deliberate attempt to silence him? We'll never know if it didn't come out. Chauvin appears to be a very cold individual, remorseless. Floyd was no angel and there always seemed to me that there was more than restraining him in the manner he was held.