Roman Republic
Around 66 BC, a movement to use constitutional, or at least peaceful, means to address the plight of various classes began. After several failures, the movement's leaders decided to use any means that were necessary to accomplish their goals. The movement coalesced under an aristocrat named Lucius Sergius Catilina. The movement was based in the town of Faesulae, which was a natural hotbed of agrarian agitation. The rural malcontents were to advance on Rome, and be aided by an uprising within the city. After assassinating the consuls and most of the senators, Catiline would be free to enact his reforms. The conspiracy was set in motion in 63 BC. The consul for the year, Marcus Tullius Cicero, intercepted messages that Catiline had sent in an attempt to recruit more members. As a result, the top conspirators in Rome (including at least one former consul) were executed by authorisation (of dubious constitutionality) of the senate, and the planned uprising was disrupted. Cicero then sent an army, which cut Catiline's forces to pieces.

Q: How many former consuls in Rome were executed as a result of a conspiracy?
one former consul