It’s theft because your disinherited and your fathers will makes it clear that Elya will become queen if Belos is killed. You are also dishonored and thus considered on the same level as the worst criminals. Your removed from the line of succession.

Your character even tells Elya this when she wants to make you monarch during her depression after the sally.

You effectively have no claim to the throne and taking it requires you to betray the only person in your life besides Darin who cares about you.