No, really: the Hebrew word 'seraph' comes from a root that means 'to scorch', or 'to burn'. So I learn from a letter to the TLS (25 Sept 09) by Yisrael Medad: 'Deuteronomy 8:15 ... "who lead thee through the great and dreadful wilderness , wherein were serpents, fiery serpents [seraph] and scorpions." This recalls the incident when, while encompassing the land of Edom, the Children of Israel once again murmured against God's plan and "the Lord sent seraph serpents against the people. They bit the people and many died' as recorded in Numbers 21:6.'
The snakey sibiliance of the initial s; the sloughing, crawling snake-in-motion sound of the final 'f'.
In this sense there is something actually seraphic about Macbeth's line: 'we have scorched the snake, not killed it ...'