A common mistake! Beatrice and Benedick don't hate each other. When the announcement of the arrival of the soldiers is made, Beatrice makes some insults of Benedick. However, Leonato calls the relationship between the two a "merry war". A happy war, in other words. They enjoy their jokes. This is shown again later in the play, after they have confessed their love. Beatrice still teases Benedick, when he goes to kiss her:
Foul words is but foul wind, and foul wind is but
foul breath, and foul breath is noisome; therefore I
will depart unkissed. (you spoke bad words, and therefore have bad breath, and can not kiss me)
Other factors show that these two have felt kind feelings for one another. Beatrice at the dance announces that she lent Benedick her heart for awhile, a "double one for his single". It appears there was love there in the past.
The difference between these two and Hero and Claudio are many. Beatrice and Benedick have obviously known each other for some time and, has been shown, most likely shared a relationship. Claudio only "looked" upon Hero before going to war, and nothing of their actual contact before their engagement is mentioned... indicating that there was very little contact. Also, Beatrice and Benedick speak for each other and to each other. Until the wedding scene, Hero and Claudio do not, speaking through others or only in each other's ears, so the audience doesn't hear. Their love seems more based on appearance, and is the work of the moment, while the love of Beatrice and Benedick is complex and has developed over time.