If she stays and tries to fight for the people of the land then she will lose several precious things in the process. She has already lost many precious lives and her near and dear ones too, perhaps she is not ready to withstand any more tragedies. This is the risk that she has to take because without it there is no other way for anyone.
Queen of Shadows and A Court of Wings and Ruin were not the concluding chapters so there is a difference in pace between those two and this particular book. This book will provide you with all the answers that you have been waiting for in the series. Aelin is going to make the ultimate sacrifice for the sake of the people that she has started loving now.