In the darkness, Marvin felt the emotional fluctuations of Muriel through the tentacles that were entwined around his body.
This proposition had a tremendous impact on Muriel. It had come to London on instinct, looking for "bait," only to encounter Marvin, who could also provide it with a long-term supply. Simply on this account, Muriel was greatly tempted.
As for being a hero of its race, that was just an added bonus. Muriel didn't have a concept of heroism; what it pursued was power, or it wouldn't have become a "Black Goat" that took the initiative to jump out of the fence.
Knowing that it was contemplating, Marvin didn't speak to disturb it. He believed that the conditions he offered were already generous enough, an irresistible temptation for Muriel.
Marvin certainly didn't want a rare supernatural being like Yog-Sothoth to fall into someone else's hands.
[I agree to your terms...]
