There's no denying who the bad guy is in A Monster Calls. The pit monster is the hungry killer, existing for the sole purpose of dragging Conor's mother to her death. When the tree monster brings his nightmare to life and Conor first sees his mother standing on the cliff, the pit monster's power is so strong that Conor can't even move (isn't that always how it is in nightmares?). And it's certainly antagonistic when Conor and the pit monster are both pulling on Conor's mother, one trying to kill her, and one trying to save her. That's about as literal a battle between good and evil as you can get.