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I looked in the OED and all occurrences of "deviled eggs" are recent. "Deviled" does not exist on its own. "Devilled" can mean "grilled with hot condiments." Neither of these make any logical sense when coupled with eggs.
But, "delved" is an ancient word meaning to dig or dig out. Since this is what happens to the yolk of a boiled egg before it is mixed with condiments and returned to the "delved" hole, "delved" eggs do make sense.
Could "delved" into "deviled" be another transposition of letters in pronounciation, like "realtor"?
Which do you use?
Grant, might this be another interesting survey.
Emmett
I always assumed it was because the yolk mixture included paprika or other spices. Wikipedia agrees:
The term "deviled", in reference to food, was in use in the 18th century, first known print reference appearing in 1786.[1] In the 19th century, it came to be used most often with spicy or zesty food, including eggs prepared with mustard, pepper, or other ingredients stuffed in the yolk cavity.
Without research, I agree with Marc completely. Paprika and mustard are often agents of Satan. The spicy connection to demons can also be seen in the Italian fra diavolo reference for certain dishes.