Right now, with no information, i am going with Drizella killed Anastasia accidentally because Anastasia was the favorite daughter and Drizella was not. If Drizella is the LGBT character, which seems possible, then maybe it will have something to do with that and Anastasia finding that out. Tremaine knows and that explains the strain between her and Drizella, but Tremaine publicly blamed Ella to keep her in check and to protect Drizella because she was her only remaining daughter. She, also, uses this information to keep Drizella in check and to get her to marry the man she wants her to marry, aka the second prince who is now the first in line to the throne after the prince who was killed died in the last episode.
I agree though that Anastasia will be brought back somehow, but i think she might come back wrong or die quickly after.