We've barely met Pablo before Robert Jordan decides he's bad news, and other characters keep repeating this like a broken record. He's a treacherous, murderous, brutal-turned-cowardly man who shouldn't be trusted. And, big surprise, they turn out to be right. Pablo's opposed to Robert Jordan from Day 1, and after losing authority over his band to Robert Jordan and Pilar, Pablo screws them all over by stealing their detonators and running off in the night. That's just what an antagonist does.
Of course, he ends up redeeming himself at the book's climax, but only partly. He's still treacherous enough to kill the people he's recruited, which disgusts Robert Jordan and everyone else involved.
Oh, and, as so many other good antagonists, Pablo's also interested in Robert Jordan's woman. Or at least that's what we're told at the beginning of the book, though nothing comes of it. It's almost as if Hemingway were trying to give Pablo all the possible antagonist cred early on but found he had so many that he forgot about one as he wrote the rest of the book.
Alternately, if you want to get a little more abstract, you could see the war as the bad guy. As we keep seeing, it's something out of the control of all the people fighting in it: it's as if they're trapped in a situation with no way out but to fight. It's the situation of war which turns people into animals, and it's the war, not Pablo, which makes Robert Jordan's love impossible and costs him his life. (It's also the war which makes Pablo into what he is, by the way). In the biggest picture, it's the war which creates artificial divisions between the combatants, who are really all human beings and very much like each other (except for maybe the higher-ups, who seem pretty bad), yet are led by war to hate and destroy each other.