He didn't just have priors, he had priors for shit like home invasion robbery of an occupied home, which is possibly the most dangerous crime it is possible to commit with the worst outcomes in terms of turning into rape, homicide, and other violent crimes. He was a bad person and was not reformed. That doesn't mean he should have been killed for what he did in this particular case, but it's no huge loss to the world and they had every reason to approach him cautiously.
That said, I think Chauvin's treatment of him once he was actually subdued was out of line and probably out of line with what is considered proper procedure, but I still think the prosecution has a tough case to make. If the defense can create a reasonable doubt that something other than Chauvin (like multiple drug interactions) might have caused his death, he wins.
I think the case against any other of the officers is even weaker, and the case against the new guy is so pathetically bad it borders on bad faith even to bring it.