The story I heard from reading Campbell's myth series of books was that snakes were originally considered powerful and wise creatures because they knew the secrets of immortality - they could shed their skin and become young again. This got wrapped up into the general Semitic set of myths and tropes, where the snake re-appears in the... Garden of Eden tempting Adam & Eve into the Fall. Eventually it and the angel 'Satan' got wrapped up into a new Manichean framework as the source of all evil and the Evil One himself, whereupon the powerful and wise aspects became negative (my good mentor is 'wise'; your evil mentor is 'calculating').

The cold part is probably just literal: I've never picked up a warm-feeling snake.