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Marco seems like a pretty chilled-out guy for most of the play. We know he's a devoted family man. Ironically, he's so devoted to his wife and kids back in Sicily, that he left them to come to America. What else was he supposed to do? He has to work. Apparently all they have to eat back in Sicily is water and bones. You probably won't see that as a new Campbell Soup flavor anytime soon. On top of the food shortage, one of his kids is dying of tuberculosis.
Other characters describe what a hard worker Marco is. Eddie's buddy Louis says, "He's a regular bull" (1.390). Louis goes on to add, "He's a regular slave" (1.392). They obviously had different ideas of political correctness in 1950s Red Hook.
We also see in several scenes that Marco's first instinct is to be a peacemaker. He constantly tries to defuse the growing tension between Eddie and Rodolpho. He does this by asserting his big brother influence on his younger bro. He tells Rodolpho to be respectful of Eddie's wishes, even as Eddie's demands become increasingly irrational.
When Eddie snitches to Immigration, all Marco's respectful peacemaking goes out the window. He transforms from Marco the mild-mannered "submarine" into the Incredible Hulk. It's easy to understand why. Eddie has basically damned Marco's family to water and bones and tuberculosis. There's a good chance they'll all die.
Beyond that, Eddie has betrayed not only Marco and Rodolpho, he's betrayed the core values of the Italian community in Red Hook. You just don't turn in "submarines" to Immigration. Most of the families there immigrated pretty recently. They take pride in helping out the new additions. Italian tradition demands that Marco get back at Eddie. If he doesn't call Eddie out, he will lose his own honor. Marco tells Alfieri, "All the law is not in a book" (1.257). When he kills Eddie it would seem Marco is on the right side of the laws that truly matter to him.