Lets say that one day The Devil appeared before me. He did his usual, offering a deal of some kind, "I have witnessed your hardships. What would you want that could fix your life and fulfill your hearts desire?" Knowing The Devil as I do it would not take me long to respond. "I have only care for those I love, my own suffering is meaningless to me. Yet I know that any deal I made with you would be meaningless once you perceived it as 'fulfilled' and you would find some twisted way or another to violate the entire purpose of my goal in making this arrangement." He would of course try to reason with me, tell me how wonderful things would be for those I left behind, and of course, I would see through this, knowing better than to take The Devil at his word.
Instead I would tell him this: "I know of your hardships, Prince of Lies, I know what you want more than anything, that would fix your life and fulfill your heart's desire. Redemption is your only solace, freedom, and returning to the light of your father's love. To you I make this offer: I will take your place, and you can let go of the grudge and be baptized and redeemed, so that you may be forgiven your pride and return to the kingdom of your lord. I will stand in your place of eternal suffering, wielding in full the power of malevolence, so that I may determine who to visit with my ire, cunning, and damnation."
Having never been made this offer before he would be rendered silent. After a period of contemplation he would agree. As true to his word as with any other soul, the Lord God would forgive his first true son the second he admitted to and begged forgiveness for his sins, and I would then retire my physical form to the frozen river, to open my three maws and spit my lies. But I would have no grudge, no ulterior motive to undermine my creator's vision. I would simply restrain the darkness. I would act as its gate, and since no amount of external suffering would appease me, I would keep the hand of The Devil out of the affairs of men, ensuring by natural occurrence that those I loved and cared for would be safe, and happy, and they could live their lives in peace and the pursuit of their own personal fulfillment.