Well there is this... I don't know how to call it, a non-linear, pathway after Ketheric is dead and you have his stone. Orin would like you to kill Gortash, and if you agree, she will not attack you while you're (pretending) to go on this mission. When you get Gortash' stone, the pact ends and you're enemies again and she will try to kill you. But this part of the pact, she tells you this will be the deal.
Gortash suggests a pact for you to kill Orin and will also leave you in peace until you do, but you can then join him to replace Orin and Ketheric as the absolute.
So it's not just search and destroy the evil ones. You can plot and plan, make alliances and betray them. The 3 chosen hate eachother, but can't kill eachother because of a "contract" but once you have Ketheric's stone, they see you as a workaround to eliminate the remaining bothersome partner.
Same goes for Raphael also. He wants to use you, but you can use him at the same time.
