While in the canon series Britannia endures a fair amount of humiliation and defeats, seeing them utterly outmatched by the Autobots and outperformed by the Decepticons brings a whole other level of satisfaction.
After several chapters of seeing Airachnid's depraved sadism and constant backstabbing (ironically of her own "side"), it's quite gratifying to watch a fed-up Megatron give her a brutal beating and force her to fall in line.
After seeing him put Lelouch, C.C., Shirley, and Suzaku through the wringer, Mao's final fate of being Shockwave's latest lab rat is incredibly satisfying.
After all the massive shit he's done in canon, Megatron both BLASTING V.V. and having him subjected to Shockwave's experiments, not to mention his Freak Out when Optimus destroys the Kamine Island Thought Elevator, is as satisfying as it is horrifying.
After what Charles did to his own children and attempted to do to all of humanity in canon, it is satisfying to see his despair at seeing all he has worked for go down in flames, as well as his final humiliation via Dark Geass before being killed by Megatron. Seeing the rest of the royal family watch their home be obliterated, their empire ripped out from under them, and reduced to being captives at the mercies of the Decepticons is pretty sweet.
Although Airachnid survives the massive Energon explosion that results from Dreadwing's Heroic Sacrifice, seeing the state she ends up in with her right arms and the right side of her face blown off, brings a level of satisfaction for everything she's done so far.
Although chilling, the Decepticons' brutal and swift conquest of Britannia brings some real satisfaction when one remembers all the atrocities the empire had committed.
Rai, Bulkhead, and Nonette repeatedly killing V.V. in Spark of Darkness. This is also notably catharsis in-universe too, as it helps Rai after everything V.V. did to him.
Lelouch finally get a major victory over Megatron and sinking the Nemesis at the end of Battle for the Ark is extremely satisfying, after all the things Megatron had done to Lelouch up to that point.
Arcee and Kallen killing Airachnid at the Siege of Castle Weisswolf most definitely counts, making sure that she pays with her life for all of her crimes. Arcee's Post-Mortem One-Liner sums up the audience's feelings the best:
Arcee: Good riddance.
In The Demon and Dragon Cometh, Megatron kills Starscream, Schneizel, and the remaining Britannian nobility by exposing them to a synthetic Rust Plague. Is it horrifying? Yes. But is it satisfying after enduring so much of Starscream and Schneizel's backstabbing behavior throughout the story, as well as the Britannian nobles' stuck-up nature?Absolutely.
It's a long time coming, but after five years since the fanfic's publication and many atrocities later, Megatron finally meets his end at the hands of Lelouch.
