Well... that doesn't make a whole lot of sense either, since it wouldn't have been infused with King's soul for the level 3 attack until after he died. Unless King kept his soul inside the blade when he was alive, but that'd be one helluva stretch. Even then, a blade specifically constructed with the intent of harvesting a soul for extra firepower would be...
Actually, I was going to say distressing if under the assumption that it was made by the mimigas, but if you give it more ancient roots... I mean, Misery constructed the Demon Crown to harvest Ballos' energy, it might not be too much of a stretch to say someone developed something similar for the fallen. 'Course, that would beg the question, did King know about this function, or did he just think of it as any old blade? If he knew, it'd make the whole "Will you... avenge me?" line at the warehouse that much more powerful.
I AM CRIPPLE AND I OVERTHINK