Duke Frederick is the wayward Duke who's the main source of trouble for our heroine. We know Duke Frederick is a bad guy as soon as we meet him – he's unseated his own brother for the dukedom, and seems unconcerned that Duke Senior now has to live in the forest like a vagabond. When Duke Frederick brings his wrath down on Rosalind, he acts much the same way Oliver de Boys (our other antagonist) does – his anger comes from a jealousy that has no basis in reason. Rosalind hasn't done him any wrong, but he'll victimize her anyway, accusing her of a potential for treachery though she's shown no signs of it. Still, there's an upside to all this inexplicable anger: if Duke Frederick hates Rosalind for what is really no good reason, then we're not surprised when he has the sudden turn-around required of villains in comedies. Who has time for bad-guys with a back-story?
Oliver de Boys has the same problem as Duke Frederick: his brother is too nice. Rather than becoming nicer, he decides the answer is to get rid of his brother. This approach makes Oliver de Boys the main antagonist of our hero Orlando. Oliver even admits that he hates his brother for no reason, but, because of his power, no reason is reason enough to murder Orlando. Oliver might be like Duke Frederick in lashing out at a threat to his power, but there's another component to Oliver's hatred: he is simply jealous. Orlando, rather than reasoning with his brother, gets angry, which means Oliver never sees any of the goodness and kindness that Orlando is so well loved for. It makes sense then, that Oliver finally becomes a good person when he's persuaded by Orlando's kind act later in the play (protecting Oliver from a lioness). Again, villains in comedies can't really be bad, because that'd be too serious. Oliver's quick turn-around shows us that Oliver was just acting out of misunderstanding (a force that often drives Shakespeare's comedies).