Why The Sith Hated Their Most Powerful Candidate

This video should be ten seconds long. The Sith hated Vader because he was the Chosen One. Done. Video over.
Except that answer makes it worse. Because it means the Sith rejected the single most powerful Force-user in recorded history. A man with over twenty thousand midi-chlorians—higher than Yoda, higher than anyone the Jedi Order had documented in ten thousand years of record-keeping. They rejected that.
So why?
And before you ask the obvious question—how can dead Sith hate anyone—let me address that right now. Clone Wars Season Six, the episode called "Sacrifice." Yoda travels to the Sith burial world of Moraband and encounters the specter of Darth Bane, a Sith Lord who died a thousand years before the Clone Wars even started. In Legends, Exar Kun's spirit haunted the Massassi temples on Yavin IV for four thousand years before attacking Luke Skywalker's students. Sith spirits don't move on like Jedi ghosts. They bind themselves to locations, transfer consciousness into objects, cling to existence through sheer refusal to let go. Death is a technicality for them. Their judgment persists.