James Blish's organization called simply "the Service" is not a Thought Police but an Event Police without being an Assassins' Guild - although it might become a Traitors' Guild (Quincunx, p. 97);
Poul Anderson's Time Patrol can also be described as an Event Police and might become an Assassin's Guild.
To complete the picture:
although Blish's flying "Okie" cities have escaped from the thought police of the Bureaucratic State, one Okie mayor is prepared to use a local Assassins' Guild "'...for the survival of the city...'" (Cities In Flight, p. 268);
as Sean M. Brooks points out, there is mention of assassins' guilds in some of Anderson's works;
after John Carter overthrew the religion of Issus and became Warlord of Mars, his next task was to wage war on the ubiquitous Assassins' Guilds.