...Souls can be eaten by hollows - monsters formed from souls that have stayed too long in the living world. And a bunch of souls can apparently be used to make the 'King's Key', the item that lets someone into the Royal Realm, but...

[He's quiet for a moment, thinking.]

I don't think I've ever heard of someone using a soul for something. As for how easy they are to take, well...

[He scratches the back of his head, looking away.]

They're like normal humans, just in spiritual form - so if you could touch them, then you could grab them and do whatever you wanted.