Tradition holds that Santa Muerte will answer the kinds of prayer requests that make other patron saints say "fuck no, asshole," but she needs some kind of sacrifice. For little things, this might mean leaving her a shot of tequila, but bigger requests require bigger sacrifices, preferably screaming ones.
With alarming regularity Mexican police find mass graves of "offerings" with images of the saint tossed among them, as well as horrible "shrines" in deserted places where everyone from children to rival cartel members have been sacrificed to her. Sometimes, the only parts of the victims found are their heads, hearts, or their flayed skin resting in Santa Muerte's bony arms.
That's enough to confirm a murder ... we hope.
Once a very secretive practice, the sheer boldness and aggression of the cartels over the years have made Santa Muerte veneration a popular phenomenon, complete with "priests," public displays of devotion, and tens of thousands of followers. And it's not limited to Mexico anymore; people involved with very shady shit have practiced it in the United States almost invariably. So if you see a smiling skeleton statue in the middle of the road on a dark night, plow right through that shit and don't stop unless you feel like supplying a new flesh-cardigan for a Mexican skeleton god.