bale·ful
 /ˈbeılfəl/  adj   [more ~; most ~]
   formal  
  1 : threatening harm or evil
   He turned and gave us a baleful [=sinister] glance/glare/look.
  2 : harmful or deadly
   the baleful effects/consequences of water pollution
  bale·ful·ly  adv  
   He stared balefully at his opponent.