Some probably thought he was drunk, others simply fell prey to the weakness that has affected many others in history, a variation on bystander syndrome. When presented with a novel situation you basically freeze up and do nothing about it because you lack the imagination to think something truly important is happening and/or you assume somebody else has acted appropriately so why would you, after all it's a crowded street, if that guy wasn't supposed to be there lying in a pool of blood he wouldn't be, right?