It was because we were Revan! In one of the greatest plot twists in any RPG known to man, we were the very Sith we were trying to destroy. .
1. The plot twist wasn't that great and was completely foreseeable if you noticed they way they kept dropping his name in the loading screens. They did it a lot which was odd seeing as he was supposed to be dead, which brings us to number
2. You weren't fighting Revan, you were fighting against Malak.