There is a character that acts openly as a friend to another character yet conspires behind their back in a way that is most unfriendly and meant to cause harm.

Notes:
This theme is not used when someone poses as a friend to someone because they merely wish exploit that person for some other purpose than to cause them harm. It does, for example, not typically apply to confidence tricks as the purpose of such tricks is generally to make a profit by any means available.

Examples:
In "Othello" (1603), sly Iago poses as confident and most reluctantly reveals to Othello details about Othello's wife, Dedemona's, infidelity. Alas, it is all ruse and fabrication carefully conceived by Iago in order to get revenge on those he envies or dislikes, including on Othello himself. It is only after he has killed his own wife in jealous rage that Othello becomes aware of what a false and malign "friend" Iago has been.