I like that he was whammied with Eb or Evanna the suspects, Eb to deal with ‘that’ once and for all and to convince Harry to see things his way on Maggie. Eb is quite speciest towards the Swartalves, and perhaps didn’t think he could get so close
Evanna would have to be Nemfected to act like this, but Nemesis has a history of infecting second in commands, they did it with the Winter Court Perhaps Thomas told her no more screwing around as he is about to become a father, prompting her to use Thomas as the instrument to take out Etri in a fit of pique.