what the actual fuck were these officers thinking?

This is a fair question, but it does have an answer: people often lie to the police about being injured in order to try and get out of restraints. It was too dark for the police to see the chest wound. Novak had a visible, but not serious, facial wound, though, and the police seem to have assumed this was the injury he was referring to.

Additionally, in this case, the actual perpetrator’s brother was the one who’d called the police. It is very unusual for a murderer or an accomplice to call the police (my understanding is we still don’t actually know why the killer’s brother did this). The police believed they were attending a one-sided assault. When they arrived, two ‘witnesses’, including the person who’d called the police, pointed out Novak as the ‘perpetrator’, corroborating one another’s stories. The police didn’t realise they were lying until too late.

I am not excusing any of the above. The police routinely treat suspects poorly and that’s what happened here. However, they did have good reason to believe Novak had committed a crime, and that belief had nothing to do with anyone’s race.