Full of deadly or pernicious influence; destructive.
source: Webster 1913
deadly or sinister
The Florida eagles have a fierce baleful look.
source: WordNet 3.0
threatening or foreshadowing evil or tragic developments
A baleful look. Forbidding thunderclouds. His tone became menacing. Ominous rumblings of discontent. Sinister storm clouds. A sinister smile. His threatening behavior. Ugly black clouds. The situation became ugly.