The Citadel Council forms the Citadel government. Their decisions have vast-reaching consequences, and they are incredibly deliberate in their decision-making. They employ the Spectres to retain peace throughout the galaxy. They are the single most important governing body in Mass Effect. They are horribly, horribly inefficient.
The Council slow plays everything. When told that one of their Spectres has gone rogue, they deny it until more evidence can be brought against him. When told that a Reaper is coming to attack the Citadel, they take the “wait and see” approach. When presented with insurmountable evidence that the galaxy is about to end, they take a coffee break.
This frustration compiles over and over again. Eventually, at the end of ME1, you’re given the choice to save them or let them be destroyed. If they were destroyed, a new council takes over in ME2 - and they hate you. If you saved them, they still don’t really like you. They really are twits, the lot of them.
Saren Arterius is a powerful Turian Spectre, who was known for his ruthless tactics, and proven track record. But he is also a rogue Spectre who allied with the Reapers to save biological life.
He figured that if sentient life bowed to the Reapers’ will, they would be spared - at least as slaves. While his intentions were originally good(ish, he’s known for his intense hatred of other species - especially humans), the effect of being in close proximity to the overwhelming evil of the ship takes a toll on him. This process is called “indoctrination” and it transforms him into a heartless, robotic jerk.