An accurate depiction
Contracts written in seemingly open speech (to avoid the evil lawyer stereotypes) that lull the person into a sense of security and leave just a few tiny, insignificant loopholes that leave their soul in her hands. Better yet, have only good consequences come about until the very moment that she drops the facade. I'd especially recommend looking into Goethe's Faust, where if you ignore the dark introduction and Mephistopheles being the devil (I assume it's not so open in your work), he'd seem like the best bro ever, getting the protagonist wenches and booze; that might be a template to borrow from, excising the dark foreshadowing at the start.
This is this.
To add, try to subvert trope after trope of what tricksters usually do. Have the trickster constantly "prove" that she wants to be fair, equitable and free of any form of coercion . . . right before you spring that last I Lied twist.
Depending on the role you want this character to play, maybe give them a throwaway victim, some random, irrelevant-to-the-plot person who's introduced just so we can see the tempter screw them over before they start buttering up a new victim. Kinda like how horror movies often begin with the monster attacking some random victims to establish their threat before the Main Characters are even introduced.
How do I make a character seem sinister to readers, and tip them off that her offers are tainted, without handing the Idiot Ball to the character who chooses to ally with her? The best I've thought of so far is to make her seem innocent to start, then slowly ramp up the evil, but I'm worried that I'll come off as agreeing with everything she says and does. (She's based off a character type that tends to be portrayed positively even if it racks up a body count.)
That's Feo . . . He's a disgusting, mysoginistic, paedophilic asshat who moonlights as a shitty writer—Something Awful