Boedhe Mac Cináeda - How did he survive Malcolm's murderous tendencies?

Private User is wondering why Boite is spared when all around him his male line relatives lose their heads:
=I mean, Malcolm II Private User... goes on a murderous rampage and eliminates, or incites the elimination of, all possible rivals to the throne. But he spares this guy. Why?

On the face of it, Boete should have been a very dangerous rival, with at least one daughter and two sons (Malcolm did not spare the sons, and probably incited the slaughter of Boete's son-in-law). But he's left alive to see his daughter remarried to an even more dangerous contender....

It's difficult not to conclude that Boete had some condition that put him - but not his descendants - absolutely out of the succession. Something as simple as a "port-wine stain" birthmark could have done it, if the old rules requiring physical perfection were still being adhered to. It wouldn't affect him otherwise, and his children probably wouldn't inherit it - but being "imperfect" he would never be chosen as King.=