Stannis Baratheon in Game of Thrones. He has his daughter, who is about 11 or 12, burned alive to facilitate a magic ritual. Which doesn't actually achieve anything anyway.
Will's mother in Goodnight Mister Tom. Abuses her son and then abandons him, tied to something, in a cupboard with his dying baby sister. Proper jaw-droppingly awful stuff.
Both of Shuggie's parents in Shuggie Bain.
There are also some utterly horrific parents in all of Sarah J Maas's books. Special mentions for Rhysand's father, Feyre's mother and both of Mor's parents in ACOTAR, Chaol Westfall's father in the Throne Of Glass books, and the father of Ruhn Danaan and Bryce Quinlan in Crescent City.