A piece of secret Marvel history that was revealed in the 2006 miniseries Illuminati was that following the Skrull-Kree War, some of the greatest luminaries in earth's superhuman community banded together in the hopes of preventing such an event from recurring.
Heroes like Reed Richards, Tony Stark, and Doctor Strange were among this initial group, and their membership has grown over the years to include many others. Together they attempted to come up with solutions to many of earth's problems, with decidedly mixed results. Thanks to them, such incidents as the Superhuman Registration Act and the Hulk's intergalactic exile were decided. Some members have seen these decisions as a necessary evil, while others had trouble going along with the group's efforts.
10 Hated: Captain America
The Sentinel of Liberty, Captain America, always seemed destined to clash with the rest of the Illuminati. Cap is the hero who famously refuses to trade lives, no matter the cost. The Illuminati is... somewhat less idealistic.
During his short tenure in the group, when Captain America realizes that they are building weapons with the potential to destroy entire worlds, he objects. Knowing he won't back down, Doctor Strange wipes his memory of the Illuminati.
9 Didn't Care: Mr. Fantastic
Reed Richards has always been a results-driven individual. One of his greatest strengths is that he believes every problem has a solution. The downside to this is that some problems are only made worse, the more someone tries to solve them.
Mr. Fantastic is unfortunately blind to this, and his drive to change the world for the better sometimes gets the best of him. This has resulted in him making murderous cybernetic Thor clones, interdimensional prisons, and worse.
8 Hated: Black Panther
We've seen T'Challa act as a wise and just king of the nation of Wakanda for years. Out of the dozens of monarchs introduced in the Marvel Universe, he's always seemed like the most sensible of the lot. As a member of the Illuminati though, he found it difficult to balance his sense of right with his sense of pragmatism.
The Infinity event opens with him among his teammates, prepared to wield a powerful weapon that could shatter worlds. Before he uses it, he pauses, telling his fellows that he is taking a moment to remember the man he was.
7 Didn't Care: Iron Man
For a long period, Iron Man was Marvel's poster-boy for the "ends justify the means" philosophy. His time as a member of the Illuminati showed him take this trait to its extreme.
This isn't to say that Tony Stark never felt guilt for the actions he spearheaded like enforcing Superhuman Registration or sending the Hulk into space, but his stubborn belief that he was doing the right thing blinded him to the extent of the damage he was doing to the world.
6 Hated: Black Bolt
Black Bolt was one of the original members of the Illuminati and took part in some of their most infamous early acts. When the question of Superhuman Registration was raised though, the Inhuman king refused to offer his support.
Like Professor X, Black Bolt remained neutral during the superhuman civil war that followed the registration act and monitored the situation to make sure it didn't get out of hand.
5 Didn't Care: Amadeus Cho
The eighth smartest person in the world, we've seen how formidable Amadeus Cho can be when he believes he's doing the right thing. For instance, in "World War Hulk" we saw him frame Angel and Hercules in order to get them to help him with the Hulk.
Cho joins the Illuminati just as Captain America and the Avengers are preparing to hunt them down and bring them to justice for their various crimes. When he breaks into Avengers Tower to steal confidential files, he's captured by a SHIELD/Avengers taskforce. Under questioning, Cho refuses to tell them where the different members of the Illuminati are holed up, telling his captors that they are trying to save the multiverse.
4 Hated: Doctor Strange
While Doctor Strange was a willing participant in many of the Illuminati's most infamous actions, he was well aware of the cost that such deeds had on their collective souls.
During the Hulk's return to earth during the World War Hulk event, he tries to manipulate the Ghost Rider into facing the green behemoth only to find his efforts unsuccessful. He explains that it is because the Ghost Rider only protects the innocent, which none of the Illuminati are.
3 Didn't Care: Namor
Prince Namor can be a cold and sometimes arrogant ruler, willing to serve the needs of the many against that of the few, which is why he should have fit in much better with the rest of the Illuminati. Unfortunately, his leveling of most of Wakanda put a damper on that.
During the Illuminati's confrontation with an alternate earth that was in the midst of colliding with their own, Namor was the only member of the group capable of pulling the trigger on an anti-matter bomb that destroys the other planet.
2 Hated: Professor X
Professor X was one of the more rational and conscientious members of the Illuminati, as seen during "World War Hulk," when Hulk returns to earth following his exile and confronts him. Xavier states that while he had no part in the decision, he would have agreed to have sent the Hulk into space temporarily to cure his condition but not permanently as the Illuminati intended. He then surrenders peacefully to Hulk, who leaves him alone after learning of the heavy casualties suffered by Mutants following M-Day.
Charles Xavier's death during AvX prevented him from taking a place in many of the Illuminati's final meetings, and it is interesting to wonder whether he would have followed his colleagues on the same dark path or be a moderating influence.
1 Didn't Care: Beast
Hank McCoy has an undeniably grand intellect. Unfortunately, that allows him to justify (or at least, attempt to justify) the tough decisions he was complicit in during his time in the Illuminati.
Following the incident when the Illuminati used an anti-matter bomb to destroy an alternate earth, Beast talks with his younger self (who, along with the rest of the original X-Men, was currently residing in the present) to talk over what happened. He attempts to justify his reasons for working with the Illuminati, only for the younger Beast to chastise him for losing his way.