Those of you who remember the original marketing materials for Warhammer Online will recall that the original poster for the game actually had a Slayer on it. You know the one; the dwarf with the bright red Mohawk and all the piercings? It's such an iconic figure in the Warhammer universe, that it seemed impossible that the game would be built without them. But, of course, that's exactly what happened.
From a game design perspective, there are two big problems with the basic Slayer concept. First, their whole purpose is an honorable death in battle, which runs counter to the protective instincts of most gamers. Second, their "live to die" attitude means that they don't really care about acquiring gold or items, which essentially removes one of the primary rewards of the MMO experience. With those two issues looming large through the early stages of development, the team at Mythic decided to focus on other careers and let the idea for the Slayer lie dormant for a while.