I think live humans is a resource far too restrictive. It restricts, as BBeast mentioned, certain keeper types. Also, above where it was all about "keeping your slaves happy" and whatnot, it would give a decisive and unfair advantage to those who decided to play as fake good guys.

If humans were to be the resource in question, I think a more fair way to put it would be to make it so it was the SOULS or something of the humans that was important. Every human, no matter how humble, has a little bit of magic in them. If you take 1000 weak measly humans and siphon their collective magic, that would result in a powerful source of energy.

So, for people like my Emperor of Peace, he would be Siphoning their magic through their worship. They may not realize it, but their prayers and faith are literally granting him power. Someone else may draw that power out by killing them all and imprisoning their souls into the very structure of their fortress, the walls literally screaming with the agony of thousands of anguished souls having their power eternally harvested. Someone who has a biological focus might require these souls to empowered their creations, soulless lumps of flesh are far weaker than those that have a soul woven into them, or whatever, so they can't just turn food and being left alone into explosive power growth. Someone who has a mechanical focus might need these souls to provide the fuel source and intelligence for their creations, the animating power and minds to their golems and machines.