After only a year at the University of the Philippines, he decided to go underground and do full-time organizing work in the factories and urban poor communities in the northern sector of Metro Manila.

At the height of the CPP split, Lagman wrote the biggest critique on CPP founding chair Jose Maria Sison's book Philippine Society and Revolution—the Counter-theses[4][5] composed of Counter-Thesis 1 (PSR: A Semi-feudal Alibi for Protracted War, PPDR: Class Line vs. Mass Line and PPW: A New-Type Revolution of the Wrong Type) and Counter-Thesis 2 (On the Reorientation of the Party Work and the Reorganization of the Party Machinery).

Lagman was ambushed and shot to death by two unknown assassins on the afternoon of 6 February 2001, at the east-side steps of the University of the Philippines-Diliman's Bahay ng Alumni in Diliman, Quezon City.

[6] At the time of his death, Ka Popoy was working on the launch of the Partido ng Manggagawa, the workers' political party that would participate in the 2001 mid-term elections.

His assassination is speculated to have been carried out by a military faction loyal to deposed president Joseph Estrada which aimed to destabilize the newly formed government of Gloria Macapagal-Arroyo.

[9] In July 2007, the Quezon City Prosecutor's Office decided to drop the case on eight suspected communist assassins since the witnesses were unable to attend the preliminary investigations.