How did the kidnappers prove that they were serious about what they intended to do to Mrs. Rafferty if Mitch didn't pay up in the novel, The Husband?

The kidnappers proved that they were serious about what they said they would do to Holly when they opened fire on a innocent man on the street, taking him out with a single, well aimed shot to the head. With the passerby's death, they ensured that Mitch knew that they were serious and that they intended his wife some serious harm if he didn't pay up.