Everything about him screams that he is a psychopath and I wouldn't be surprised if he enjoyed torturing Sia because her pain was a source of entertainment for him. He could also be bipolar where he did love Sia but the littlest things would set his anger in motion. His pocket watch is a sign that he is a guy who is hell bent on vengeance and he counts down the time of those who anger him...
Isa's theory could possibly be true that Om and Mannat know the whereabouts of the real Sia and they've kept her alive so that her property isn't given to a trust fund but since they have Tara, they can kill the real Sia which is why Mannu went to Balli.