A backyard drug cook tests a new formula on her willing friends. It produces hallucination turning them into murderous, sadistic adversaries. She must survive their attacks as they slip deeper into madness.

The big difference being that the Kevin Spacey character - Keyser Soze was reviled as such only at the very end. Up to that point as far as the audience is concerned he was a loveable small time criminal that dealt in petty crime without harming anyone in particular.

This female MC would have to follow on the same vain of mystery and scope for most of the film before reviling she in fact induced the psychosis.

In The Usual Suspects Kevin Spacey's character was legendary world wide as the devil incarnate this revelation in the end gave his character a great enough of an impact for a moving conclusion. In this story the MC is loved by her group of friends for the drugs she gives/sells them. If this is going to be reviled at the end of the story in the same way as with The Usual Suspects it won't have a significant enough impact to be as moving.

If you are modelling this after The Usual Suspects then perhaps best to devise a character who's story cold have a greater scope.

Could she secretly be the head of the largest drug cartel in the world?

Exactly. I think females can make better dark characters. Mostly they are written as insane and dark. I think they be "bad" without being mad.

Thanks for the help.

What would sell the concept for me is not the story line, but the character, the portrait of an unapologetically amoral, irresistibly charming, intuitively manipulative and brazenly deceitful young female shapeshifter who has already mastered the art of wrapping other people (particularly men) around her little finger. IOW: she is not likeable, but she is compelling.