I'll skip the spoiler tags, since anyone wanting to avoid spoilers would have stopped reading this topic by now.
Damian thought the player was intent on destroying Ygerna, but allowing that plan to proceed was still his best option to get to the Hall of Echoes. His plan would have been to wait until the path was open, stop the player and resurrect Ygerna himself. Obviously his follow through on that was too late.
Enough people disliked the ending of D2:ED (especially before FoV was announced) when it was 'you were fooled, Damian is stronger and now you are imprisoned', imagine the reception of 'you probably put at least 40 hours into this, but Damian killed you, he is now invincible, the end'.
