Martin is a bit of a rabble/rebel-rouser. He works as a journalist with the anti-Monterist press in Sulaco. When Montero's people appear to be on the brink of victory, he approaches Mrs. Gould (a.k.a. the wife of one of the most powerful men in Costaguana) and floats the possibility of the Goulds and their allies throwing their weight behind a plan to have the Occidental province secede from the country.
Martin is not especially political by nature, but being in love (with Antonia, the daughter of one of the Goulds' close friends) makes him pretty interested in figuring out a way to achieve peace. Basically, he wants to make war so he can make love, not war:
Even Decoud himself seemed to feel that this was as much as he could expect of having her to himself till—till the revolution was over and he could carry her off to Europe, away from the endlessness of civil strife, whose folly seemed even harder to bear than its ignominy. After one Montero there would be another, the lawlessness of a populace of all colours and races, barbarism, irremediable tyranny. As the great Liberator Bolivar had said in the bitterness of his spirit, "America is ungovernable. Those who worked for her independence have ploughed the sea." (II.5.77)
He's a lovelorn peacemaker (and also kind of a bigot) who just wants to carry his bride away to peaceful, lily-white Europe.
Having gotten the Goulds on board with this plan, he flees Sulaco with Nostromo with two aims: He wants to get a large supply of the Goulds' silver out of town before the revolutionaries storm in, and he is hoping to go get General Barrios and bring him back to help defend Sulaco. Well, actually make that three aims: he's also just trying to avoid certain execution at the hands of the Monterists (since spearheading the anti-Monterist press made him a big target for them).
He and Nostromo encounter some hiccups in their little plan and aren't able to get the silver entirely away from Sulaco. As a result, they decide to bury it on the Great Isabel, a nearby island, and have Martin chill out there until it's safe for him to return to Sulaco.
Unfortunately, he goes a little nutty being alone there on that island with just the silver and, thinking that Nostromo is never coming back, decides to shoot himself. He steals just enough of the silver to weigh down his body so it sinks. This poor lovesick revolutionary ends up as just one of many casualties related to the silver.