Hmm I thought of Alexandar more as a prophet, than a biological heir of Lucian. Maybe one day he heard voices in his head and decided that this is his "true" heritage(This is how some "prophets" even came to power in real life)?
Maybe Dallis is his mother (She seems to be a dragon, as you can see at the of this video:
https://www.youtube.com/watch?v=PKwL7p3xK_s&feature=youtu.be ) and she told him all that hogwash so her little boy can feel better about himself? He IS crazy enough to believe all this and after losing Lucian the people of Rivellon would believe everything, if he can keep the Black Ring away from them.
As for his death: I don't know if the Hall of Souls is still part of the story, but maybe we will meet him again in the third act as an undead and he will make us pay for our decision to kill him? Or he delivers some exposition to explain some of the more cosmic stuff (fighting gods, Void, Demons).
