Felipe is correct.
3.3.4 Terror. ... Select any spaces where the executing Faction has at least 1 Underground Guerrilla; pay 1 Resource per City or Department (0 for LoCs).
PROCEDURE: Activate 1 friendly Underground Guerrilla in each selected space....
4.4.2 Assassinate. Assassination ... may occur in any of up to 3 spaces selected for AUC Terror (3.3.4) this Operation in which AUC Guerrillas outnumber Police.
PROCEDURE: In each such space, remove any 1 enemy piece.
Thematically, the assassinations are unavoidably terrorizing people. Already active guerrillas are already busy doing something else or too exposed to local authorities to get away with assassinations.
Contrast this standard case, however, with the "Limpieza" Event, a sort of extreme expression of assassination:
Ruthless elimination: An Insurgent Faction executes free Terror with any Guerrilla, removes any 2 enemy pieces in the space, and sets it to Passive Support or Opposition (unless 0 Pop). The Terror places 2 markers.
Because the Event text says "with any Guerrilla", you may select an Active one (5.1.1).