mercenary (n.): A source who is working for us in exchange for money (typically discreetly delivered cryp) and detailed explanations of where all their body parts are likely to be found in the event of a contract default. A pleasure to work with, relatively speaking, because all the cards are on the table.
idealist (n.): A source who is working for us and betraying his putative employers for ideological, religious, patriotic, etc., beliefs. Widely considered unreliable, especially compared to mercenaries, since anyone who’s turned their coat once may do so again if they change their mind. Consequently, the first step in handling an idealist is to secure a firm yet invisible grip on their gonads, or failing that, a half-gram of KL-8 implanted in one of their heart valves.
– excerpted from Rilial’s Informal Dictionary of Intelligence and Security Terminology