"We're really quite nice and friendly, but everyone has a beastly side to them, don't they?"

In this quote, Sid Vicious is acknowledging the duality of humanity. He is saying that while people may appear nice and friendly on the surface, everyone has a darker, more beastly side to them. This might describe the inherent human impulses and desires that can sometimes lead to negative or destructive behavior. Vicious is suggesting that this beastly side is a natural part of being human which it exists within everybody. By acknowledging this, he is advising us to be aware of our own potential for darkness and to strive to be kind and compassionate towards others.