Atrocious
Fiendish
な adjective
A villain who is bad is the most atrocious and fiendish villain of them all.
きょうあく
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
誰もが、その凶悪犯が刑務所で大人しく刑期を務めるのは無理だろうと思っていました。
Everyone believed that the atrocious criminal would not fare well in prison.