The snake has two facets. When it sleeps as the kuNDalinI shakti in a person it only promotes lust. Even the little prANic energy in them is lost in that lust. Therefore the snake in that situation is said to be poisonous and evil. It is this poison that Garuda and the peacock destroy. When the poison is eradicated, the same snake of the kuNDalinI, when it rises to the sahsrAra cakra, it pours forth nectar! Now it is a ‘good’ snake - not the corresponding Tamil word, which means a deadly snake!
The fact that energy is lost because of lust is accepted by all. But the same energy becomes immortal energy (nectar-flow) by yoga is not generally accepted by people; and that too, by the people, who have never done the experiment!