I know this is off topic, but Satan didn't exist when the OT was written, by the way.

http://www.biblicalarchaeology.org/daily/biblical-topics/bib...

Not quite true, or at least not so baldly. Job describes the "satan", or accuser, in a manner that clearly anticipates his later depiction as "the accuser of the brethren" in Revelation. And while not providing a name, the descriptions of the mythical falls of the kings of Babylon and Tyre in Isaiah and Ezekiel seem to draw on legends similar to the later Christian elaboration of the story of demonic rebellion and exile.

I might be wrong, but AFAIK the old testament Eden story features the serpent, but has no mention of Satan.

The identification of Satan with the serpent is a Christian and Islamic thing.

The verse most specific to a "built in" reaction to snake/snake like patterns (per OP) is Genesis 3:15:

"I will put enmity between you and the woman, and between your offspringe and her offspring; he shall bruise your head, and you shall bruise his heel." (ESV)

I think we are onto something here.