I am sure a great many before have posed this very question. Well now I am here to say what they are. They are men. Men, like your blocky character, who have traveled to the Nether, although not through portals. No, these men have died by the countless creatures outside the Nether, and have been forced to join the ones inside. These men, these people have done what oh so many have, and oh so many have never come back from doing. In Minecraft, there is no heaven, there is no hell. Once you die, it is over. Once you die, you go to Nether, you are forced to join the ranks of the undead horrors that lurk below your blocky feet, your blocky mountain, and your blocky fort. They wield the golden sword. They wield it because it is weak, but shiny. These men were not strong enough to survive in the world, and are being punished, and they show it. Their constant screaming, screaming at themselves for not going to the gym before they set sail. Screaming because they couldn't survive. Why is Notch so cruel to the dead? We may never know, same as how many licks it may take to get to the center of a "tootsie-pop".
Alas, another peasant fooled by Notch's words. My good fellow, when you press the Respawn button, you simply go to the Nether. Notch then places another bewildered traveler on the island to test their strength and will to survive.
My theory was similar, but not the same. In mine the zombie pigmen are travelers that entered the Nether without backup obsidian or flint and had their portal blown out. Living in the Nether for a long time, they gradually grew adapted to the fire, learned to make tools with the firestone thing and the lightstone, found other pigmen, and, since they had nothing to eat but magical pigman meat, they eventually mutated into one. Of course, most just died. They can't talk because it's impossible to talk in Minecraft single-player, and can do nothing but make grunts similar to those of the player.