By all accounts, Pablo's not good news. Not good-looking, either. He's "dirty," "stubble-faced," with small-closely spaced eyes on a large round head. Robert Jordan's fond of comparing him to a swine. And even more often than we're reminded of Pablo's porcine (a highfalutin word for "pig-like") appearance, we're reminded he's a drunkard, cowardly, not trustworthy, and potentially treacherous. It's kind of shoved down our throats by everyone all the time. Not that it needs to be; it only takes Robert Jordan a look at Pablo to figure that out. Barely having met him, he concludes: "You do not know how he was before; but you do know that he is going bad fast and without hiding it." (1.217)
And so it's no surprise that Pablo proves to be the primary source of trouble for Robert Jordan: Pablo doesn't want his band to help our hero, and even forbids him to blow up the bridge at all (before Pilar takes charge of the situation); he's really obnoxious and provokes confrontations that almost turn lethal, using alcohol as an excuse; and, of course, he eventually steals Robert Jordan's supplies and dooms the band to certain failure (before un-dooming it by coming back). Plus, he's got a slightly creepy thing for Maria, and is not happy to her and Robert Jordan together. Que pasa, man? What's the deal?
The million dollar question is, how did Pablo get this way? It turns out that a long time (i.e., a few months) ago Pablo was a different kind of guy. Not a sullen drunkard, but a skilled and ruthless guerilla leader who loved a good fight for the sake of the People and the Republic. We learn from various characters of some of the daring missions Pablo led, and of the large numbers of fascists he killed. Check out the Pablo we see in Chapter Ten, Pilar's story of how he "liberated" their town at the start of the war. He made short work of those fascists, but was enough of a believing populist to arrange the killings so that all the townspeople would share responsibility for them.
On the other hand, Pablo's energetic participation in the war also seemed strongly fueled by something else: he just really likes to kill people (at least if they're fascists). He's at the top of the heap so far as bloodlust goes: he tells Pilar after the killings in her town that he enjoyed seeing every one of the fascists (mostly civilians) get massacred, except the priest, who, as he was being hacked to death by sickles, did not die with enough nobility.
But when we meet him, Pablo's different. He doesn't want to do anything anymore, except drink and spend time with his horses. Helping Robert Jordan would "make trouble," and be too risky for himself and his band; that it's important for the Republican cause doesn't matter to him. It's not that Pablo suddenly supports the fascists – that's not it at all. He just doesn't want to do anything that would put him or his band in danger, and that means basically not doing anything. In that respect, he's a foil for Robert Jordan, who's willing to give anything and everything to the cause unthinkingly, perhaps too much. But why the change, Pablo?
We get various theories from other characters on why Pablo has changed. Anselmo (and Robert Jordan) thinks it's because he captured his horses: actually owning something for himself that he cares for makes him reluctant to risk his life. Instead, he wants to "enjoy life."
Rafael thinks he's finally been traumatized by all the people he killed, and can't bear to kill any more. That's worth considering, since there is that moment in the book when he announces that he's "filled with sorrow" at all the men he killed and says would restore them all to life if he could. Then again, he was drunk at the time, or pretending to be drunk (we're not sure which would make him more believable).
For understanding Pablo, though, we've got to go with Pilar, who thinks he's been overcome by a sense of great sadness and hopelessness. His fear of death is part of it, and his sudden clinging to life (through the horses) is a symptom, rather than a cause. His turn to drink is also a symptom, since it's a way he has of escaping; Pablo doesn't drink to be happy. We think all the killing he's done has probably taken a toll and him, and is another part of the sadness.
The quickest way of saying it, though, is just that he's sick of everything about the war and wants to be done with it, but sees there's really no way out. Pablo's early patriotism probably took a big blow when it became clear the war would last much longer and be much worse than anyone expected, and the Republic's strong early position quickly came to look more troubled (we know from Pilar that the fascists recaptured their own town three days after Pablo took it).
Pablo's very perceptive – he's the only one sharp enough to perceive right from the start that the bridge will be trouble – and it's possible he feels (rightly) that the Republic will lose. He's unlike Pilar, in that respect, whose almost religious "faith" in the Republic keeps her afloat, but that's precisely the issue. Pilar admits that her patriotism is an "illusion," such that she can genuinely believe the Republic will triumph. Pablo cannot. This is all speculation, though. As the different opinions of the characters attest, Pablo can be understood in many ways.
The other big question about Pablo is whether he's just out for himself or not. Various band members seem to think he'll do anything to stop them from taking the bridge, including blowing them up with grenades while they sleep. Pablo, on the other hand, claims he has their own interest at heart, and is trying to keep them from something they're too stupid to recognize will be fatal.
Ultimately we've got to go with Pablo on this one, though it wouldn't be easy to argue his intentions are always pure. Nevertheless, as nasty as he seems at times, if you compare what Pablo says he's doing to what he actually does, they kind of jive. He never actually hurts anyone in his band, nor gives any sure sign that he's going to. He doesn't steal the detonators and doom his "friends" so they will be killed; he does it because he hopes that will stop them from trying. When he realizes it won't, he comes back. He'd felt unbearably lonely without his band, whereas by contrast he recognized that he'd actually felt animated the day before when he did help them by leading off the patrol. So Pablo does have a heart. Sounds a bit like Donne's "No man is an Iland," huh?
Still, before you decide that Pablo's actually not a bad guy, keep in mind that same interest in "his own" leads him to almost casually kill the five guys he's rounded up for help, without seeming too troubled by it. It's almost as if he's kind of happy to do it, from the way he keeps hinting at what he's going to do to Robert Jordan. Even if he's a victim of being "misunderstood," Pablo still gives us the creeps.