While in college, like many youths in his time, Marsman joined demonstrations denouncing Philippine involvement in the Vietnam war and criticizing the abuses of the Marcos administration. He was also active in the student council at the University of the East. His organization was the Alpha Phi Omega fraternity.
Back home, he courted danger when he became openly critical of the powerful local warlords who maintained private armies and terrorized the local people.
Marsman was in his final year in college when Marcos declared martial law in 1972. Elder brother Heherson fled to the United States where he lived in exile for several years. But Marsman and the rest of the family stayed behind.
Quietly, Marsman took up assignments for the anti-dictatorship underground. He joined a small group that clandestinely brought city activists in and out of the countryside in Isabela and nearby provinces. He collected medicines from relatives and put them in boxes with false bottoms to be taken to militarized areas for wounded villagers and the guerrillas. Word spread among his relatives that Marsman had “joined the subversives.”
Soldiers picked him up one day in 1974, and the following morning, his broken body was found in front of a churchyard, mutilated and unrecognizable but for his clothes. His face was bloated, his jaw displaced, most teeth pulled out, his tongue slashed. He had stab wounds in the neck.
Marsman’s killing was denounced by Manila Archbishop Jaime Sin, to which, then defense secretary Juan Ponce Enrile reacted by claiming the New People’s Army (NPA) was responsible. Enrile said the NPA had found out that Marsman was a government agent doing undercover work. Witnesses however later identified the persons who took Marsman away as National Intelligence and Security Agency (NISA) agents. The regime’s role in the senseless murder was confirmed further when government agents came to convince Marsman’s family to accept money in exchange for their silence, an offer the family furiously rejected.
Marsman’s mother later wrote that Marsman was killed because her elder son, Heherson, dared to defy the dictatorship.
Marsman’s townmates turned out in huge numbers for his funeral march, silent proof of their sentiments. But the series of family tragedies drove Marsman’s parents into severe depression. Marsman’s father Marcelo died from a heart attack a few months later.