Beatrice's characterization in Much Ado's Act IV scene i is quite conflicted. Her cousin Hero is about to marry Claudio when instead of marrying her, Claudio accuses Hero of sleeping with another man the night before. On this happy occasion, Beatrice's cousin is slandered at a time when a woman's reputation of purity before marriage meant everything, and if she was thought impure or unchaste, no honorable man would want her, which is why Hero faints upon being accused and then is rushed away to safety.
It is here that Beatrice switches from joy to anguish. Only recently have she and Benedick admitted they love one another, but Benedick is one of Claudio's men while Beatrice is kin to the ruined Hero. Though on the opposite side of the scandal, Benedick admits that he sides with Hero. "Surely I do believe your fair cousin is wronged" (IV.i.1913). Because of this, Beatrice asks Benedick to prove his love to her by killing Claudio, the man who falsely accused Hero. At first Benedick refuses. "Ha! Not for the world" (IV.i.1942). At which point she says, "You kill me to deny it. Farwell" (IV.i.1943).
She loves Benedick, and he loves her, but she loves her cousin more. She feels a stronger loyalty to her cousin and either clearing Hero's name or avenging her ruin. Eventually, because Benedick's loyalty winds up lying with Beatrice, he agrees. "By this hand,/ Claudio shall render me a dear account" (IV.i.1977-78).
Beatrice asks Benedick to prove his love by avenging her cousin, claiming that if he doesn't take action against Claudio, he can't truly love her.
To sum it all up, Beatrice is quite conflicted in this scene between her love for her cousin and her love for Benedick, but her true loyalty lies with her family.