A monster is not evil. It does not understand its actions; it lives by simple instinct. It does not choose its actions, they are a string of reflexes responding to physical stimulus. A shark is not choosing evil when it bites a person's leg, it is simply hungry. It does not understand what it is biting, it responds by reflex to the vague shape of a sea-turtle. An act of evil requires both knowledge and freedom. But someone ruled by outside forces cannot choose. Someone consumed by their own suffering cannot understand the world around them. Any creature with sufficient knowledge and freedom will choose good, making evil something that cannot exist in reality.