At the end of the scene, Beatrice and Benedick are left alone on the stage (at line 255). They have not, as yet, spoken to one another about the changes in their feelings. Beatrice is angry with Claudio and Don Pedro for treating her cousin Hero as they did, and she makes the following comment:
Ah, how much might the man deserve of me that would right her! (260-61)
Beatrice is as emotional and as irrational as Leonato had been. To Beatrice, the only way to make matters "right" for Hero is to get vengeance. But Beatrice does not have a close relationship with Benedick at that point, so she does not feel that he could be the man to help her get that revenge.
Yet Benedick declares himself to be her friend and offers to help her even before he knows what exactly she wants done. Benedick then confesses his love to Beatrice:
I do love nothing in the world so well as you. Is not that strange? (266-67)
This is certainly the strangest avowal or declaration of love ever made in literature. Benedick lacks wit and the poetic words of love. Rather, his words are plain and ordinary. More importantly, Benedick is shocked at what he is saying. He is not only asking Beatrice if it is strange, but he is also wondering about the strange change that has occurred in himself. He still cannot quite believe it.
Beatrice responds with words that are equally prosaic and odd, but she does also declare that she loves Benedick. And Benedick is so pleased with her response that he repeats his offer to help her before he knows what it is that she wants him to do:
BENEDICK Come, bid me do anything for thee. BEATRICE Kill Claudio.
BENEDICK Ha! Not for the wide world. (286-88)
Benedick's offer is typical of any lover, but Beatrice's request and Benedick's response are both unexpected and surprising. In this splendid example of verbal irony, the lover's promise to "do anything" becomes humorous because in reality the promise is an empty one, more often than not. Such promises usually do not really mean anything.
The reader should also be mindful of the close bond between soldiers like Benedick and Claudio. These two men are sworn friends, closer than brothers, who are willing to die for one another. Beatrice's request is asking Benedick to break this very special and sacred vow. Benedick's response reflects that special vow of camaraderie between two soldiers.
Beatrice, though, then declares that Benedick does not really love her, and she starts to leave. Benedick, torn by his own emotions, asks Beatrice to stay and be his friend. But Beatrice argues that if he were truly her friend, then he would fight her enemy, namely, Claudio.
Benedick's love for Beatrice is too strong for him to resist. For that love, he agrees to act contrary to his usual habits. No other force in the world could make him act in this way except love. He agrees to "challenge" Claudio (line 325). Benedick agrees to fight a duel with Claudio and perhaps even kill Claudio. The influence of love, Shakespeare reveals here, is so powerful that it can make one change his deepest convictions or beliefs and break his strongest vows.