The undead (souless) that is made by Black Rose kills should disappear (most of them) after the battle. But they count as killed when the battle is over. It's feature (or rather a game mechanism)
Note that Dark Summoner's Demonic Blood Spear and Vampire Beast bite (not its claw) do the same. As well if the soulless makes a kill will raise another soulless.
But the soulless are pretty weak to say at least.
You can check the nosophoros wiki (far from finished yet) for more unit features:
http://dom3.servegame.com/wiki/Nosophoros