Sombra and Indigo were best friends. Luna's personal students loved to explore the world together and learn all they could about magic. Sombra, however, pursued a different path of magic than Indigo. As chancellor of the Crystal Empire, and captain of the Royal Guard, the two didn't spend much time together in later years. Perhaps if they had, Indigo could've stopped Sombra. Stopped his rise to power. Stopped his tyrannical reign and the Great Northern War. Stopped the slaughter. But he didn't, and couldn't.