The death of M-24. 'Nuff said. It's also accompanied by a detailed flashback of what M-21 and M-24 suffered through when being used as experimental material by the Union. Amazingly, it's treated as a happy memory, as M-21 remembers his bond with the other members of the M-series. Then we're treated to M-21 bawling his eyes out as he says that he doesn't want to loose M-24, and another flashback of M-24 saying that he thinks he might have had younger siblings before he became an experiment, and asking M-21 if he'd be OK with calling him by the Korean term for big brother.
Rai's entire existence. He's spent thousands of years living in isolation, the only special thing he had was to gaze out the window of his mansion. And then he finally gets to live in a world with friends and companions, and in less than a year he's already dying.
The deaths of Urokai and Zarga, where we get treated to a flashback where they always wanted to entertain and impress Rai.