On July 21, 1979, a 14 year old named Edward Hope Smith became the first one to fall victim to the series of deaths. Edward was living in one of the poorer neighborhoods, filled with garbage and crime. He had attempted to run away from there, but did not succeed. Edward was also an extremely enthusiastic football fan, and was training at the time to play for his high school's football team. On the night of his death he was spending time with his girlfriend at a local skating rink. He would set out to walk back home after, but, sadly he would never reach his destination. Edward was likely a target because he was a child, by himself at night.
The last child victim of these murders was a boy named Timothy Hill, but the actual last victim was a 28 year old by the name of John Porter. Porter did not make the list of killings until Williams was traced to his death by a fiber. He lived with his grandmother and was kicked out multiple times for his odd behavior. It should also be noted that Porter was severely mentally sick, and spent an amount of time inside a mental hospital. He was found dead in April, 1981 due to multiple stab wounds. His mental state would explain why he was an easy target, because Williams likely would not have killed him if he was a man who wasn't mentally unstable.
Sources: http://www.trutv.com/library/crime/serial_killers/predators/williams/begin_2.html