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.