Undead Creatures are neither dead nor alive. Some of them were certainly normal men and women at one time, but they have long since died, and in many cases, they walk through Tibia as mindless monsters, terrorizing adventurers and killing anyone who approaches them. The regular corpse isn't indirectly turned into an undead. Those who are turned into undead were probably manipulated by a deity to become a soldier in an endless war against the living. Because these creatures are not alive, they are not affected by Life Drain, and usually Poison does not bother then at all.