The gang are not malevolent. They are acting out the will of the universe. At night, the leader takes the chalk from the safe place, and blindfolds the chosen one. They are given the chalk and spun around in an anonymous alley, and then they draw.
Each time, they draw an outline.
They walk away solemnly. In a few days the outline will claim a victim. It is rarely, if never in the same place. Somewhere in the universe a body appears in the same sprawl. A murder. A heart attack. A suicide. It doesn’t matter. When the police draw their outline, it is exactly the same as the one the gang drew.