It was supposed to be in contrast to the Doctor. The Delgado/Ainley Masters were very suave and controlled against the Doctor's somewhat chaotic nature. I think psychopathic!Master was the idea as counterpart to the Doctor being kind?... It didn't really work out though, since against Tennant they both had the same sort of manic energy. And later with Time Lord Victorious it kind of destroyed any counterpart thing that could have been going on. Really it was as if Ten had been picking up tips from Simm!Master, which I'm pretty sure wasn't the original intention.