Cruel, Coldhearted, Relentless, Ruthless
noun, な adjective
A cruel cool person is more cruel and coldhearted than just a normal cruel person. It's probably because they're cold all the time and it makes them mad. You should stay away from people like that.
れいこく
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
あなたのこと、ずっと冷酷な人だと思っていました。
I always thought you were a cruel person.