The Collaborative International Dictionary of English v.0.48:

malignantly \ma*lig"nant*ly\, adv.
   In a malignant manner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

malignantly
    adv 1: in a malignant manner, as of a tumor that spreads

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "malignantly":
   balefully, banefully, corrodingly, corrosively,
   counterproductively, damagingly, deleteriously, detrimentally,
   disadvantageously, disserviceably, harmfully, hatefully, hurtfully,
   in spite, iniquitously, injuriously, invidiously, malefically,
   maleficently, malevolently, maliciously, malignly, meanly,
   mischievously, nastily, noisomely, noxiously, ornerily,
   poisonously, prejudicially, spitefully, toxically, venomously,
   viciously, virulently, wickedly, with bad intent,
   with malice aforethought, with malice prepense