King Uther's account can be settled slowly.
Now Duncan has the advantageous condition to actively declare war. In the future, even if he plans to kill King Uther, no one can say anything.
To be honest, King Uther being moved by Igraine's beauty is nothing unusual. In this turbulent era, it's only natural for the strong to possess beautiful women. However, he cannot act this way; it's somewhat shameful and not the demeanor of a king. If he had directly defeated the Duke of Dingtiger on the battlefield, or even killed the Duke of Dingtiger, and then taken Igraine as queen,
no one could have said a word against it!
Indeed, Igraine herself would have accepted her fate, as it is normal for the victor to take everything, including the wives and daughters of his enemies.