The police took Garner to the ground because he was resisting arrest for an offense he had been arrested for in the past, so one cannot claim he did not understand that he was breaking the law. And the cops did not kill him, his own health problems did. He lacked the cardio to walk well - wrestling with cops is vastly harder on the heart. So once again he knew he was in bad health (something the cops could not know) yet still chose to put up a fight.
So the cops did not chose to make this a capital offense, as your post insinuates. Garner made his own choices and paid for them with his life.