Tear Jerker: The Downer Beginning, with Kouji losingliterallyeverything before meeting Minerva-X. To wit: his father was murdered, his girlfriend was raped AND killed, he killed the culprit (his grandfather, of all people), lost an arm... and then not only he became a messed-up Woobie, Destroyer of Worlds, but he and Mazinger ended up literally destroying Earth. And for worse, Minerva witnessed everything and couldn't do anything to help.
Hell, any time that ZERO is activated can be considered this. To elaborate this is a monster that was once a hero and guardian of the human race. Sure it was stated that it could become a god or a devil, but most of the time we've been shown the heroic side to it. And then to see it just...degrade itself into what amounts to a selfish all-powerful child who's scared of being forgotten is heartwrenching to say the least.