In Pablo's band, Rafael is "the worthless one." He's not dedicated, he's not disciplined, and he's not dependable, though it can be said for him that at least he's not a complete coward. He likes to ascribe it all to being a gypsy.
He and Pilar, the other character with gypsy blood, have a difficult relationship; she can't stand how worthless he is, and likes to say so, too (remember her opening line). Who else would wander off while on sentry duty in pursuit of a couple of hares he wanted to stew (or wanted somebody else to stew, rather), thereby letting a mounted patrolman slip by and almost cause the band to be discovered?
Also big on the things-that-infuriate-Pilar list, Rafael doesn't dig the Republic so much; he's not interested in the cause, so much as "making revolution." Revolution's fun to him. He gets to kill people (which he enjoys), blow up trains, and eat fat hares and drink wine at the end of the day.
Not to mention, he can be treacherous. Although Rafael doesn't have the courage to do it himself, he wants someone to kill Pablo. Why not Robert Jordan, who he approaches with the idea after having known him what…three hours? That's why everybody calls Rafael an animal.
If one wanted to look at it more sympathetically, Rafael likes to enjoy the "little things" in war, like food, wine, and singing (with guitar).