Willful means "deliberate" or "stubborn." A child who exhibits willful disobedience knows she is doing something wrong (even if she tries to convince you otherwise).
While being full of will, or determination, doesn’t necessarily seem like a bad thing, the word willful is negative in meaning. Use it when someone is behaving in a stubborn or uncooperative manner. To describe someone who possesses dedication or perseverance in a positive sense, consider determined, driven, or decisive — and that’s just the d's!