Its about someone thinking they know whats good for someone else and being overprotective, he says "Just stay with me, safe and ignorant" meaning just be my pet, I know whats best for you, you don't, he also says "I'll be the one to protect you from A will to survive and a voice of reason," those arent things you protect someone from and neither is, "Safe from pain, and truth, and choice And other poison devils," those arent real poison devils, I think its about someone controlling someone else, making them think they are there to help but all they want is a pet to do what they say and not question, hence the title "Pet"