The sith teachings are all about embracing ALL of your emotions, yess that usually means you embrace Hatred and anger, but it can also mean love, compassion. The Jedi teach you to supress your emotions which is why ani turned darkside because he embraced love, which turned to hatred when his wife was dying. So yes there can be light side sith.
In a way, besides embracing love, he embraced his hatred when he went all Rambo on the Tusken Raiders. Doesn't help that after his mom died he became obsessed with the idea of saving people from death. Enter pregnant padme.