Why Did Euphemia Have to Die?

She shares the same fate as she did in the anime, where she had set up the Special Administrative Zone of Japan only to have it end in a disaster as she was accidentally Geassed by Lelouch to kill all the Japanese. Euphemia was later killed by Zero, who used her actions to stir up the Black Rebellion.

Why did Lelouch lose control of his Geass?

So after all that I think Lelouch lost control over geass because of mental instability. And he lost control over his geass right before he was gonna make order to Euphemia. That takes agency away from Lelouch and makes it not a punishment for his abuse of his gift, but an evil plan of a bad guy.

Did Lelouch use Geass on Euphemia on purpose?

So in episode 22 Lelouch accidentally uses geass on Euphemia and casuses her to order a mass slaughter. So instead of having character build up and plans matter we will now have Lelouch slip on a verbal banana peel at the exact wrong time for no properly explained reason causing a mass slaughter of civilians.