how do you think serial killers are able to keep up appearances and be so articulate with their words which is the exact opposite of what you would typically expect?

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!

Personally, I believe that Hollywood itself is to blame for giving society a blueprint, in a sense, of how a serial killer "should be". They say they've got killer eyes, hard/cold personality, defensive body language, etc.

But in reality, we're shown time and time again that killers are like everyone else. They're  our neighbors, our friends, our family members... people that live life day by day like the rest of us and yet, perceive their wrong doings as if it were genuinely part of a hidden life.