Why is Max Damage Incorruptible? Because one of the primary recurring themes of Irredeemable is that part of being a hero is accepting that not everyone will always appreciate what you do or trust you for it, especially when you inevitably make a mistake. Since Max was originally a villain, he already expects people not to trust him and so is emotionally prepared for rejection the way the Plutonian and Cary aren't. He's not incorruptible because he's striving to be morally perfect, he's incorruptible because he has the trait of real heroism that the Plutonian lacks. Basically Max is a hero despite acknowledging his flaws while Tony is a villain to deny his own.
The Plutonian is seen to have a father whos upright and strong, if slightly resentful towards others, and who breaks down upon realizing he couldnt save his wife from a terminal illness and kills him and her. This capacity carries on to his son, on a more godlike scale...
Why does the Plutonian agree to leave Coalville alone out of respect for Max Damage? As noted under the Heartwarming entry, Max's sincere gratitude to the Plutonian is perhaps the one thing that can move Tony. All of the ungrateful masses muttering behind his back and its one of his most hated enemies that can genuinely say "thank you" to him. It's all Tony ever wanted.
On another level, it's possible that Tony not only figured out that for all their bad blood he and Max understood each-other in a way others didn't, but they were also kindred spirits as godlike men alienated from humanity. Tony even noted how Alana telling Hayes Bellamy secrets of Max was like how he himself had his secret identity as Dan Hartigan outed by her to their coworkers.
Qubit comes off as cold-hearted and detached at times, but this is a story about superheroes getting emotionally unstable and carried away. His cool-headedness makes him sane enough to fix the situation, not perfectly but as best as he can.
He also actively seeks to redeem the Plutonian throughout the story; he knows very well that a lot of the Plutonian's actions came from the inability to deal with people, and sets in motion a plan that will restore the Plutonian's heroism, but also mercy kill him since hes done too much for anyone to ever accept him again.
Qubit is not the only one who understands the Plutonian's situation. Hornet, Tony's best friend on the team, noticed that Tony was going to crack under the pressure eventually. And in a later volume, the American President muses that maybe the Plutonian had one really bad day; as the leader of a nation, he is probably fully aware of the pressures of dealing with huge responsibilities.
A mad scientist has a one-use portal that can take one and only one person to another dimension, keyed to a world with absolutely no supers. Max Damage attempts to use it to send his sidekick out of the Plutonian's reach forever, but she mistakes the gesture as a combat maneuver, and throws it at her opponent... a rampaging killbot promptly sucked into that paradise, and now irretrievably stuck there with unsuspecting and defenseless billions.
To be fair, the robot seems to be a fairly typical one as weapons go. It doesn't seem to be anything that a rocket launcher couldn't handle. Furthermore, it might be remote controlled and will just shut down when it's there, or at least eventually run out of power. It's not like Max dumped the Plutonian onto our homeworld.
Going by what happened in the main Irredeemable plotline, that entire world is probably now slated to be conquered by the Vespa on account of being sold out by Hornet along with every other peaceful world Qubit discovered. Which is an entirely different sort of Fridge Horror.
Modeus knew what was coming, and planned for it by hiding out in Samsara. He told nobody, and allowed Samsara's innocence to shatter and abused Tony's paternal feelings for the boy, because Modeus thought this would help Tony to return his affection.
Considering that Max Damage was willing to even kill children back when he was a villain, what OTHER atrocities did he commit when he was evil!?