Here's why:
* It describes a noun: You would say "a sinister plot," "sinister motives," or "a sinister smile." It tells us more about the noun.
* It doesn't act like a verb: You wouldn't say "I sinister the plan."
* It doesn't function as a noun: You wouldn't say "The sinister was scary."
Let me know if you have other words you'd like to categorize!