Quote Originally Posted by factotum View Post
It was purely a new series thing. In fact, I don't think the Master was ever obviously insane before--he wanted to rule, that's always been the case, but he was quite methodical in the way he went about doing it.
Quote Originally Posted by Friv View Post
Well...

He was methodical, but he was super-murderous too. As soon as anyone started getting in his way, he would kill them, regardless of whether it was necessary for his plans. He was insane in a classic villain sort of way - a polite psycopath. IIRC, his more manic persona in the new series was meant to reflect Tennant's more manic Doctor.
He did however have little ability to distinguish fact from fiction, famously having believed the Clangers to be real alien life during the story "the sea devils"- to which there was a callback involving the teletubbies in the more recent series (this second part shouldn't be a Curly spoiler...)