BOSTON—Carlos Arredondo has seen tragedy before. His elder son was killed in combat in Iraq, and his other son hanged himself nearly a decade later. Now, he is haunted again, with flashbacks from witnessing the Boston Marathon bombing, but this time he has support from the man whose life he helped save.

Jeff Bauman Jr., 27 years old, was cheering near the finish line when the bombs went off. Suddenly, he was on the ground, maimed and thinking he was going to die. Then a stranger—Mr. Arredondo—rushed to him. "The bystander...