I would say that maybe that´s actually the reason they are so articulate - being usually quite intelligent and great with words. Most of the serial killers have a high IQ and do well at school. Not everyone of course, but most.
Also we have to remember that serial killers are not people you can spot instantly - and that´s probably why people are so quick to judge them as being maniacs, psychos and lunatics. It´s easier to deal with it all if you can just brush the possible reasons behind it all aside!