A monster is the existing evil of one's person's heart that fears those around him. If anything, it is a title of those whose wickedness has gone to the point of no return, in which cannot be forgiven, and with no means for the monster to redeem himself. It is not the same as a kind person who poses himself as a bad guy—reasons being, to save someone's life and change the world. A monster has already lost its common sense, rationality, and poses threat to everyone. And since we're all humans with mixed similarities to an animal, there's no point in pointing what a monster usually looks like.
Moreover, the monster can be an intellectual or an obsessive killer—depending on what that monster usually was when he was a child. In psychological, the monster is typically yourself—yet you're unaware of that fact—you travel along with a world that doesn't exist. It makes you understand the guilt, and test if you can take them—saving yourself or becoming a monster if you give up.
Usually, it isn't the appearance that makes a person a monster. Take, for example, Kira from Death Note; he poses himself to save the world by changing it using a Death Note and kills every criminal until he started losing his purpose by killing civilians. His actions become unjustified, and his mental state being torn apart—a sign of becoming a monster. At this point, he has overwhelming power with an unrivaled genius that poses a threat to be called a monster.
Now then, philosophically speaking, what is a monster? I think you know that a monster is more than what you're speaking about. A monster is a by-product of a human who experienced extreme despair, suffering, and pain in this world. As you know, if there is no human in this world, then nothing is happening. Animals and insects don't cause anything in the world, moreover, it is us that harness their capability to destroy the world.