Even if what the Devil offers you seems perfect, it's still corrupted inside. By accepting the offer, you become corrupted inside too. This is because the Devil doesn't want to befriend you, but to fool you into becoming a slave by giving up your soul for something you believe to be worth it at the moment. As soon as you give up your soul, the offering starts showing the corruption, but it's too late, because you paid the price already and there is no return policy. You pay with your mental sanity and your life. Don't deal with the devil.