Stickman by Dennis E. Yates
While waiting for his girlfriend in a mall parking lot, Doug witnesses a suspicious individual scoping out cars. The prowler breaks into a van, and instead of stealing items left behind he waits for the family to return and carjacks them.
With no time to call for help, Doug decides that he must stop the carjacker from harming the family. He's also horrified by the possibility that the carjacker could be the notorious serial killer dubbed "Stickman."
Hot on the trail of a killer headed for privacy, Doug must decide quickly how far he is willing to go.