Modeus using Bette Noire's body and enhanced gravity powers to beat Tony to a pulp and then raped him.
In a bid to protect humanity from the Plutonian's eventual wrath, Hornet made a deal with the Vespa alien race to provide the coordinates for every Earth-like planet his team has ever visited in return for their assistance in neutralizing the Plutonian. Considering there were Earth-like planets with sentient life, his choice condemned countless lives to slaughter.
The very premise itself. The Plutonian started off as a smiling and selfless hero, always ready to do good, fight evil and save the day... but there was always something bubbling under the surface and it takes a series of really awful events to make him feel heroism was no longer worth it. His sheer power and strength makes fighting him futile, and he has no compunctions with who he kills. The populace is shown to be utterly terrified and suicide has become commonplace.
To put it another way: Imagine if every nightmare scenario that Lex Luthor and Batman could come up with for if and/or when Superman went supervillain came true. Only, instead of world domination, "Superman" wants to just kill everyone, everywhere.