falsely

traitorously, treacherously, deceitfully, faithlessly, falseheartedly, behind one's back, disloyally, underhandedly, basely, unfaithfully, perfidiously, dishonestly, erroneously, fallaciously, wrongly, incorrectly, unjustly, unscrupulously, roguishly, knavishly, under the garb of, hypocritically, insincerely, dishonorably, crookedly*.

Antonyms truly*, justly*, honorably.