These mutants could be regarded as the advanced forms of zombies. In fact, they were already dead from the virus infection, but they were not undead like zombies because they were not transformed by death power. However, only necromancers could feel death power, so it was impossible for those besides necromancers to distinguish the difference between them and ordinary undead.
Roy's demon blood could transform others into liches, and Frostmourne could also transform others into undead. But Roy himself did not currently have any death power, so it had to be said that it was a rather strange situation.
Seeing this, everyone might also be able to imagine that it would be very easy if Roy wanted to integrate death power as a new source power because the affinity between his body's attributes and death power was so high!