1) A person that works in a malicious and sinister way, to his/her own end, not caring who it affects.
2) A person that acts in a sneaky or conniving manner, to further their own ends, to the detriment of others and, all the while, denying any wrong-doing.
Combining the word snake and weasel to produces the ugliest, ratiest, fakiest, liar that your will ever meet in your entire life or if you believe in that stuff lives.x