Slaves are wounded and tortured souls roaming Hades without purpose. Hades slaves are subjected to an eternity of mutilation, branding, eye-gouging, and limb breaking.
Zombies have different weapons, ranging from nothing to a pick-axe. Luckily for you, the weapons don't make any difference in the zombie. Zombies are easy to kill, and they never deal too much damage. They don't seem to create massive swarms like the skeletons do.