Dark magic is never a good thing. Sombra didn't know this. Indigo didn't know it either. That's why he didn't try to stop Sombra. To stop the war. To stop the slaughter.

Celestia was not the only one who took a personal student. Luna did as well, even as she changed into Nightmare Moon. When she fell, so did he. 1,000 years later a unicorn wakes up in the Everfree with only a small idea of who he is.