The sicarii were a sub-branch of the zealot sect of Judaism. Their religious understanding evolved into a distinctly anti-Roman agenda, which brought about their development into a near-guerilla organization. "We must destroy the Roman Empire. We will kill all collaborators, no King but God." was one of their many (for lack of a better word) slogans. The sicarii derived their name from the knives they were infamous for carrying with them. Typically, they would wait until a large public celebration, a time when the streets would be crowded. They would sneak up behind a Roman official (or Roman collaborator), and assassinate them. The ensuing panic would give the assassin opportunity to escape. The sicarii became such a menace that the Roman army had to intervene. Anyone suspected of being affiliated with the zealots was put to death.

{Jewish Sects and Orders}

THE ASSASSINS or SI.CA'RI.I.
As the Zealots represented the fanatical extreme of the Pharisees, and the Galileans were a class of the Zealots, continual disturbers of the peace, so the Sicarii, assassins, or "murderers" (Acts 21:38), formed an association or secret society of bandits, springing out of the Galilean rebellions. These fanatics, carrying a small poniard (sica, whence the name), concealed in the folds of their dress, attempted to carry out their purposes by secret assassination, visiting Jerusalem at festival times, and inflicting mortal blows unseen in the crowds on those whom they adjudged the enemies of God and His people. At other times they levied open war against the Roman authority.