There is a difference between an animal and a human being. Yet, human beings are animals. Monsters are fictional, but there is a real difference between a monster and a person. And yet, some fictional creatures cross the blurry line and others are seemingly both at once. In the real world, categories are rarely absolute or binary, and they aren't anymore so in fiction, as much as we might like things to be clear, simple, and absolute.
I'm at a loss to help you further understand the difference, blurry as it is, between a monster and a person. To me the concepts are as simple as the difference between an animal and a person, despite some creatures in the Monster Manual crossing or blurring the line between the two.
Why bring it up? You can do things to animals you should not do, ethically, to people. You can do things to monsters that you should not do to people. In the real world, that's often how we justify committing evil upon others, is to "dehumanize" them, to consider them other than or less than human.