The Collaborative International Dictionary of English v.0.48:

Harmful \Harm"ful\ (h[aum]rm"f[.u]l), a.
   Full of harm; injurious; hurtful; mischievous. " Most harmful
   hazards." --Strype. --Harm"ful*ly, adv. -- Harm"ful*ness,
   n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

harmfully
    adv 1: in a detrimental manner [syn: detrimentally,
           harmfully, noxiously] [ant: harmlessly]

Moby Thesaurus II by Grady Ward, 1.0:

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