Resentfully tells you how someone does a thing when that unfair-angry feeling is inside it. You might agree resentfully: you say yes, but you are not willing. You might look at someone resentfully, or hand something over resentfully. The action still happens; the feeling rides along with it. It is the adverb of resentful, so it is about manner, not about naming the feeling as a noun. Though actually, a resentful yes is often more work for everyone than a clear no, because the job gets done with a chill on it.