sinister

"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";

"black deeds"; "a black lie"; "his black heart has concocted yet another black deed"; "Darth Vader of the dark side"; "a dark purpose"; "dark undercurrents of ethnic hostility"; "the scheme of some sinister intelligence bent on punishing him";