One of my design goals for atlantean hobos was to incite "dark magic" and all kinds of reprehensible behavior without having "spells" per se.

So I started from a theme of diminishing Vigor, which is true for all people but especially true for the eponymous hobos of this game, and I added the concept of magical items that counteract that diminution. Then the question of how the magical items are made. The obvious answer is that there's a price to make something that sustains one's Vigor, and that price is the Vigor of somebody else. With diminishing returns, so you never get as much benefit as what it cost.

I think the brief rules on these points will suffice as motivations for truly despicable behavior either by the player's hobos (hopefully not) or by non-player characters (great plot hook). A background moral question: given what the players know about the origins of these magical items that sustain their characters, is it acceptable that they use them?