This whole week has been full of "you know this is being filmed, right?", and still, STILL, the deeply ingrained racism of this country makes people think they are going to be just fine. In the case of Chauvin, it's because he's gotten away with it many times already.
I think's its that and I also wonder if the cops assumed the phone video wouldn't get as much detail as it did and they could play it off as a heart attack or an accident. I saw one of the different angles of the shot and they were all so calm. These were four sizable men with guns, they had lots of options, but they were just waiting him out until he died. I think they were trying to kill him.