Garner had a stress induced heart attack. If he hadn’t died then, he’d be dead by now anyway.
As an EMS professional ( ER physician and former EMS medical director) I was shocked at the slow and incomplete response the EMT’s took in the case. In the second video which shows the EMS response, Garner looks like he’s unresponsive and probably not breathing for 3 or 4 minutes after the EMT’s arrive and they aren’t doing any of the things I would expect them to be doing ( listening for breath sounds, checking a pulse ox, putting the pads on to check his cardiac rythym, putting him on oxygen etc. If they had reacted more quickly the guy might have survived.
I suspect he suffered either a primary Myocardial Infarction, or a respiratory arrest, followed by MI. Just putting a guy that big down prone can cause his O2 sats to drop like a rock.