"Only the good die young"—that's what Billy Joel sang. And, in On the Waterfront, it seems to be true.
First, we see the saintly Joey Doyle get shoved off a roof. Then, Kayo Dugan, another longshoreman, steps up to bat and offers to do the same thing Joey wanted to do: testify to the cops about what Johnny's really up to. He's doing something morally courageous (though it took a little encouragement from Father Barry). When Johnny's thugs attack the meeting Barry holds at the church, and start beating people up, that's enough for Dugan.
Unfortunately, punishment falls on him swiftly: Johnny gets word from informers in the police department that Joey. As he's working on boat, unloading crates of whiskey, Johnny's henchman cause a bunch of crates to fall on Dugan and crush him, making it look like an accident.
Poor, flat Dugan.
But this give Father Barry the opportunity to give his great speech, saying that what happened to Dugan was a "crucifixion." Dugan becomes another example—along with Joey—of a person who tried to do the right thing and got killed for it.