A manager is responsible for the safety of customers within a store. For example, the floor should not be slippery and goods should not fall on customers. Also goods sold must not be dangerous when used. The incident you describe in which a manager verbally abused a complaining customer, who during the argument suffered a heart attack, indeed, was unfortunate. However, I doubt that the manager could be held legally responsible for the customer's heart attack.
Irate customers test the good manners and goodwill of store managers.
Store managers, who cannot control their temper, would be smart to take anger management training and possibly courses in empathic listening and assertiveness. Dealing with complaints is not easy. Prevention of complaints is easier than correcting them. That means treating customers with respect and having goods and service of high quality. Above all, savvy managers are attentive to what customers want and demonstrate their concern for the customers' interests.