Sunset Shimmer tried to kill the Humane Six in the EG movie, Chrysalis imprisoned both Cadence and Twilight with intention of proably starving them; plus she killed a cat in the comics, Nightmare Moon would've killed numerous ponies by blocking out the sun, It's even implied that the Teenage Dragons would have done something horrible to Spike; if the ponies hadn't intervened.
I can't help but imagining a picture where instead of Cadence snatching away the Crysal Heart and Spike from Sombra, but Rarity yelling "MINE!!!". Her love of jewels giving her superpony leg strength to make that leap xD
If he was ever to return, I hope he would do it at the helm of a massive dark crystal golem army. Imagine the squishy sounds coming from the clash between these killing-machine of dark power and the Royal Equestrian Army.