When the creatures attack Kane in his cave, they don’t use the vampire’s great strength or advanced speed, instead “the talon-like nails of the black were tearing at his face…” This is classic zombie tactics against the living. Interestingly, during the entire course of the story, there is not one mention of fangs. From all textual clues, it appears that the creatures possessed normal human teeth. (There is a mention during a later fight that the creatures were “sucking” at Kane’s wounds. While this is a possible mention of fangs, this fails to account for why there was no mention of this earlier.)
During this same fight, Kane makes the observation that the creature’s skin is hard as wood; later the creatures are described as “mummied monsters.” It is highly unlikely that vampires would have wood-hard skin, their unique form of immortality allows their bodies to continue replicating skin and blood cells—thus keeping their skin live-human smooth. Zombies lack this, and are consistently running down their walking corpse of a body as it decomposes ( Brooks; Zombie Survival Guide, pages 10-11). However with proper conditions, like an African desert, the zombies could have been naturally mummified by the shifting sands and winds.
Later, Zunna tells Kane that “Men and beasts flee them [the creatures]…” There is no record of animals fleeing vampires; most animals either treat them as living humans—though occasionally a dog will be mistrustful of a vampire. Zombies create terror in their local ecosystem as beasts flee to escape the virus.
After N’Longa takes over the body of Kran he tells Kane “Them vampires no talk nor yell; they dead.” As we know, all vampires talk; most seem to have an affliction that forces them to talk for pages, making these silent vampires almost unvampiric. Zombies are unable to talk; the only sounds they can issue are, at best, a low moan. Later it is mentioned that the creatures emitted “silent gibbering[s].” According to the dictionary “gibbering” is defined as “Unintelligible or foolish talk.” These two references do not apply to vampires, but quite handily match up with the moans of a zombie—what are moanings if not unintelligible talk.
When Kane and N’Longa cause the creatures to notice them, they attack in a hoard. “Out of the cave they came swarming, the terrible black silent shapes; up the slopes they came clambering, and their red eyes were turned toward the two humans who stood about the silent city. The caves belched them forth in an unholy judgment day.” Vampires rarely attack as a gang, and when they do, their numbers rarely reach over five or six. This, however, is a classic example of zombie behavior.
Finally, during the climatic fight where the creatures overrun Kane biting and tearing at his flesh, we find evidence that the creatures are incapable of feeling pain; as Howard records “skulls were shattered, their faces caved in and their limbs broken” yet they still continued to attack. If a vampire were given this rough treatment he would be forced to retreat until the great pain abated and he was physically ready for another round of combat. Zombies however, have been known to continue attacking even once more than 80% of their bodies were destroyed—and continued attacking until intensive trauma was induced on the brain. (As for the argument that Kane shattered the creature’s skulls, look again to the text. Right after “shattered skulls” Howard mentions “faces caved in,” it seems Kane was limiting his attacks to the front part of the head—dismembering their faces—thereby destroying the zombie’s sense of sight, smell, and (limited) communication, but doing little, if any, damage to the brain itself.
At this point in the article, I am willing to state that the creatures are unarguably zombies mummified by the African environment. Despite my wishes as author, a few pieces of evidence going against my findings need to be dealt with.
During his first encounter with the creatures, Kane manages to kill the things by stabbing them with his cat-headed wood staff. It would seem obvious that the wood killed the creatures (giving good evidence that the beasts are vampires) but I don’t find this the case. Kane is first shocked by this event—one so knowledgeable should not be stunned by finding a wooden stave useful against a vampire; second, Kane rationalizes that the magic within his staff is what killed the “vampires.” I am inclined to agree with him.
Several times in the short story, the creatures are noted as possessing red eyes. In truth, I can find no explanation for this phenomena (Max Brook’s The Zombie Survival Guide helpfully notes: “The eyes of a zombie are no different than those of a normal human.”) Red eyes only seem to exist in the skulls of vampires and demons; one category of beings with red eyes remains untapped for an explanation—normal humans afflicted (in life) with albinism.
It is possible that a thousand years before Kane arrived at the lost city, a number of albino children were born, and due to the fear of the city leaders were only allowed to breed among themselves, after several generations (and a lack of fresh albino children from the normal population) the slightly inbred albino population was allowed to intermarry with the normal population. This caused an explosion of albino population, and thus created a civilization of albinos. 
These albino zombies may provide explanation for one of the most curious events of the story. During the climatic battle, Solomon Kane’s flesh is bitten and “sucked” by the creatures many times. Kane is not turned into any type of monster and continues on his adventures. Some types of vampires need several feedings to transform a victim into a blood-sucker, but as shown above, the chances of these creatures being normal infective zombies is low to nil. From this we can conclude that the zombies in “Hills of the Dead” are inflicted with a noncontaminatory strain of the Solanum virus. (Note: the Solanum virus is the official name for the zombie plague, as divulged in the Zombie Survival Guide.) Perhaps the albino genes altered the Solanum virus into a genetic cocktail that can only be contracted by other Albinos.
One final puzzling note is worth pointing out: when Kane first encounters the creatures they exhibit both an attraction and fear of his fire. Neither vampires nor zombies have a fear of fire. A vampire would laugh, or at the very most back away, if a torch was waved in his face; a zombie would fail to recognize the fire and set himself alight—slowly burning himself to death. This is one of the more puzzling elements of the story, and the lone factor that could make a future researcher assign the creatures as a completely different type of monster.
Despite several odd features, I fully believe that the creatures Kane encountered in the African savanna were zombies. This historic encounter would dovetail nicely with the recorded history of zombie attacks recorded by Max Brooks (in his Zombie Survival Guide), where a reasonable argument is built that the Solanum virus progenated somewhere in the African continent.
Brooks, Max. The Zombie Survival Guide. New York: Three Rivers, 2003. Print.
Howard, Robert E. "The Hills of the Dead." 1930. The Savage Tales of Solomon Kane. New York: Del Rey, 1998. 223-53. Print.
National Organization for Albinism and Hypopigmentation. "NOAH — What Is Albinism?" The National Organization for Albinism and Hypopigmentation. Web. 22 June 2010.