Wilful is a British spelling.
Willful has a double L in American English.
Aren’t there times when you write a word, but it doesn’t look right? Then you erase and write it a different way, but it still doesn’t seem correct. Almost everyone has experienced this phenomenon at one time or another. It often happens with words that may or may not have a double letter, such as willful . . . or wilful. You stare at the word, asking yourself whether it has a double L or not. It doesn’t have to be a mystery; the first place to turn is the dictionary.
According to Cambridge Dictionaries Online, wilful describes things that are intentionally bad. For example, you might say that a mischievous child’s disobedience is wilful. Wilful people are determined to do what they want even if they know it’s wrong. So, you could also refer to a deliberately mischievous child as wilful. This same dictionary also identifies wilful as a British spelling. It appears in printed news journals and in literature. Here are two British examples:
“There is, I believe, in every disposition a tendency to some particular evil, a natural defect, which not even the best education can overcome.”
“And your defect is a propensity to hate everybody.”
“And yours,” he replied with a smile, “is wilfully to misunderstand them.”
—Jane Austen, Pride and Prejudice
Following an investigation by Police and Fire Service it was established that the front door of the flat had been set alight and as such the fire is now being treated as wilful.
While Britain and other English-speaking countries favor wilful with a single L, you will almost always find willful in American writing. It’s not impossible to find an example of wilful, but it’s viewed as a misspelling. Notice how willful is used in these quotes:
The girl staring back at me was angry, powerful. This girl and I were the same. I had new thoughts, willful thoughts, or rather thoughts filled with lots of won’ts. I won’t let her change me, I promised myself. I won’t be what I’m not.
—Amy Tan, The Joy Luck Club
Still, Every Song Ever acknowledges that even when only a few finger motions are required to sample new sounds, breaking out of the familiarity trap entails an act of will—by which Ratliff doesn’t at all mean an act of willful elitism.
—Spencer Kornhaber, The Atlantic
Thus, the mystery is solved. Willful and wilful are variants of the same word. If willful looks right, you probably grew up in the United States. However, English speakers from around the world relate better to wilful. Choose the form that will be best understood by your audience. If you forget, you can always use a synonym such as deliberate, intentional, or conscious. If you are referencing a person, headstrong, obstinate, and self-willed are good options. Or you just might demonstrate that you possess the characteristic in question and use whichever form you please!
from Grammarly Blog