he had no plan to ressurect Ygerna
Yes, he did. In addition to Lucian's comments about that in the end cutscenes for D2:ED, Damian says to Ygerna "I battled tirelessly to release you from death's grip."
(or at least not now, and the dragon knight was not in it).
So Daimian couldn't have seen an opportunity to take advantage of the dragon knight's actions, what with his own resurrection attempts apparently not close to success?
In any case, killing the player character as soon as a possible threat was perceived would have made for a pretty short game.
And once the Patriarch opened the passage, Damian had no use for the Dragon Knight anymore.
Presumably Damian would have tried harder to kill you if you meet him after the passage to the Hall of Echoes was created and then unsealed.
