Here's one to get you started: When a very small number of people have everything and a great many people have nothing, not even what they need to survive, those hungry desperate masses will have nothing to lose and every incentive to take what they need by using any and all means.
Next, try to think about what might be potentially lost when large numbers of people are needlessly dying. What contributions great or modest those lives might have made if they'd been able to survive.
I'm not suggesting you should be personally responsible for providing for every person's basic needs, but it doesn't take a great amount of creativity or insight to come up with pragmatic reasons why we might be better off collectively if we work collectively to minimize the number of people around us who are struggling with the basic necessities of life and well-being.