The place where it became manslaughter over just restraining a suspect is when Chauvin paid more attention to the crowd than to the man under his knee.
Floyd might have died on his own in the back of the cop car on the way to the station or ER, given what else we know about his health at this point.
But it was Chauvin pulled him back out of the cop car, instead of hustling to the nearest ER or station to get back up/medical help.
He let himself get distracted from his duties when handling suspects and people under his physical control. I get that druggies can lying, resist, and spaz out without being in a health crises, but I also get that being a cop means you are supposed to be trained to ignore distractions like a crowd of people with phone's while handling a resisting or possibly in physical distress suspect.
None of this absolves Floyd of his crimes either, and frankly the idiot effectively killed himself while under police control, though not intentionally.
Neither Chuavin or Floyd were or are heroes; frankly they are two idiots who's collision has utterly changed the face of the US political landscape, whether we like it or not.