WordWeb Online

1. Marked by deep ill will; deliberately harmful
   "viperous gossip"; "took viperous pleasure in...watching me wince";
   - poisonous, venomous, vicious, malicious
    
2. Of or pertaining to a viper or vipers; resembling a viper
   - viperine

See also: beady-eyed, bitchy [informal], cattish, catty, despiteful, leering, malevolent, spiteful, venomed, vindictive, vixenish