marked by deliberate deceptiveness especially by pretending one set of feelings and acting under the influence of another; "she was a deceitful scheming little thing"- Israel Zangwill; "a double-dealing double agent"; "a double-faced infernal traitor and schemer"- W M Thackeray
Double-dealing is behaviour which is deliberately deceitful. Marriages were broken and lives ruined by the revelation of double-dealing. = betrayal, duplicity