Backstabber noun – One who betrays a trust or an allegiance. Snake in the grass and backstabber are semantically related in traitor topic. In some cases you can use "Snake in the grass" instead a noun "Backstabber".
Snake in the grass noun – A deceitful or treacherous person. Backstabber and snake in the grass are semantically related in scoundrel topic. You can use "Backstabber" instead a noun phrase "Snake in the grass".