That could be but the killer leaves a trail by doing this. For example, people that know the perp may notice that he is out of town for a number of days while killings happen in another region. The perp may leave a credit card paper trail in his new location or, on the other hand, use cash where a credit card would be more appropriate thereby arousing suspicion. I suspect the perp's name is already on a very long list of suspects. When the perp changes his routine by going outside the DC area to kill he narrows down the list greatly.
I really think the chance is good that this Sicko will be caught if he resumes his assassinations--no matter where he goes.