>>cheald+(OP)
First degree does require prior intent, but it can be in a very short period of time (minutes, or even seconds). Basically, if you can stop and think about what you're doing at any point and prevent the murder, you can be convicted of 1st degree murder for having carried it through.
I think there could be a pretty compelling 1st degree case there, since the officer had Floyd's neck under his knee for so long. He could have stopped the assault at any point.