Actually, both in the third and fourth games, there are some inconsistencies in the design of the factions. It seems the developers had problems with mythology or their imagination wasn't working well enough. But... these are just words, and we have to play with what we have. It's unlikely that anyone would redesign the factions – and then the balance would change, and players' habits would be disrupted.
The Lich became the Venom – perhaps it would have been better to leave the Lich – the functions are the same. But two weak factions were combined into one, and if the Lich were left, it would disrupt the balance within the city between the undead and the living (demons).