When Siris opened the tomb,we know he regained some of Ausars memories. And we also know that every time Siris rebirths, The Dark Thoughts get stronger. I finally think TDT are getting to Siris. My reason is this: when you kill The Iron Hunter, Siris seems outraged and annoyed as if Ausar is there, not Siris. Ausar is finally coming out and Siris doesn't like it, obviously, but I think this thought overpowered Siris. IMO TDT said WEAKLING, HE DOESN'T DESERVE TO LIVE. HE SHOULD DIE. And then Siris killed him. To explain when you spare The Iron Hunter, Siris overpowered the thoughts. To people that haven't read IB:A this may not make sense.