darvom
Doesn't that soul have Azure Dragon's daughter in it? I can see why he'd try to sell him on it in the fashion of "Please save my daughter when you eventually do find it, because you will. Absolutely you will." Meng would absolutely save his daughter if she's spent all this time trapped, that's horrible after all!
if i'm not mistaken the only way to use the sword is if she recognizes you as her master, your former wearer was not recognized by the sword so every time he used it he used a ton of vitality that allowed him to use the sword 3 times before to die, it is clear that in normal cases no one would be able to use the sword, but it allowed to be used but did not recognize him as his master.
28th00:Also, isn't the weakness on that insta-kill sword that it uses 1/5th or 20% of your lifespan, each time you use it? You can use it 5 times, but the fifth time will kill you.