Here's the thing, if nobody filmed this or made a fuss about it, how likely is it that Chauvin would have faced any consequences? Zero. I'm not saying all cops are racist, or the use of force against black people is always unnecessary, but I do think this would've been swept under the rug. But there is a record for how long he knelt on Floyd's neck. And, the reality here is, it doesn't matter what a POS Floyd might have been, cruelty, neglect, and physical abuse are not appropriate from a cop.
Now, all that being said, I think it is bad he got shanked. I also am surprised that guards would allow it to happen, unless he was a mouthy punk who demanded special treatment...But, the US prison system is FUBAR, and lord knows it needs to be reformed...