Councilor Udina's attempted coup will no doubt be analyzed for generations to come, but a clear picture is beginning to merge. Udina had contacted Cerberus to coordinate what was intended to be a bloodless takeover of the Citadel, in which he would force the other councilors to grant him emergency powers so that he could command the Citadel Fleet. He would then direct the fleet to liberate his homeworld, Earth.
The plan fell apart early when Executor Pallin and the salarian councilor caught wind of it. In defense of the plan, the Illusive Man dispatched his top assassins, commanded by Kai Leng, to kill them. Udina had little choice but to support the assassins with an armed force sufficient to hold the Citadel. Captured confidantes have indicated that Udina and Leng's alliance was relatively fragile: Udina may have planned to turn on Cerberus once the fleet was his to command, and Leng departed when he calculated that Udina would not succeed.
Persistent rumors suggest that Udina might have been a high-functioning victim of Reaper indoctrination. His actions played right into the Reapers' plans: even if the coup failed, it would damage Citadel governance. If it succeeded, his plan to retake Earth would likely have turned into a military blunder that Council forces could ill afford. However, there is no direct evidence of his indoctrination, nor obvious opportunity. It is more likely that Udina acted out of desperation, and in doing so, cost humanity its councilor.