creep or slime-ball
Thesaurus.com lists creep as a synonym for scoundrel. But it lists a lot of words as a synonym for scoundrel. Why do I pick creep?
In the discussion which has been moved to chat, the OP says, disagreeing with the suggestion of backstabber:
A person who steals an old lady's savings by hacking into her account
is not a backstabber, but certainly a scoundrel, and the act can
certainly be classified as a "dick move".
Such a person evokes the same shudder as something slimy that has crept out from under a rock. (My apologies to everything that lives under a rock.) But this is ELU, and why should anyone believe what I say? So here are definitions:
creep, from Oxford Dictionaries
A detestable person.
‘I thought he was a nasty little creep’
As for slime-ball, which I prefer: Oxford Dictionaries:
A repulsive or despicable person