He was unlikely to have been found guilty of 3rd degree murder anyway, so the radical antisemitic cop-killer-supporting AG probably decided to take a chance: if he can get a conviction despite not having a case he might as well make it a good one, and if he can’t then it’s no worse to lose on a 2nd degree charge than on a 3rd degree one.
And no, most sane people do NOT believe it was murder at all. The evidence indicates that the knee on the neck did not contribute to Floyd’s death, which would have happened no matter how he was restrained. He died of a heart attack, brought on in part by his poor health, in part by being high on a large dose of fentanyl, and in part by the stress of being restrained by the police — which they were 100% correct to do.
The worst Chauvin is guilty of is failing to take sufficient care of a person he knew to be in medical distress. Maybe he should have checked on him more frequently. But he’d already called an ambulance so even if he’d checked on him it’s not clear what he could have done that would have made any difference.