Insidious, sinister, or pernicious? All three of these words have decidedly unpleasant meanings, each with its own particular shade of nastiness.
(ɪnsɪdiəs )
adjective
Something that is insidious is unpleasant or dangerous and develops gradually without being noticed.
"Sinister" means threatening, evil, or foreboding, suggesting something bad, harmful, or dangerous. It can also refer to something that is evil or treacherous in a mysterious way. Historically, the word was also used to mean "on the left side".