Sorry, didn't mean to shoot the idea down. Assuming this is a homebrewed world and the monsters the are just supposed to have a human and monstrous form then there is an interesting take.
Make the world a place where monsters and magic used to be common, but magic is dying out, and with magic dying it's becoming harder and harder for monsters/fantastical creatures to reproduce. This gives a reason for random monsters to find each other and form a group, they are trying to save magic and all monsters. They shapeshift into human form in order to survive because they have to live within human society as there is no where else to go. The trick in this case is making them actual villains since the party likely has the same goal of fixing magic. The monster villains are trying to accomplish it through widespread human death, and the PCs and some friendly shapeshifters are looking for other ways.