It's a thriller about a psychopath who's killing people. In the end it is revealed who the killer is.
It's a movie is about a guy killing people.
It's a movie about a crazy person killing people.
kyouheiさん
ご質問どうもありがとうございます。
様々な表現があると思いますが、いくつか自然な表現を紹介します。
1. It's a thriller about a psychopath who's killing people. In the end it is revealed who the killer is.
--- psychopath = サイコパス
--- to reveal = ～を明らかにする / ～を明かす
2. It's a movie is about a guy killing people. And in the end you find out who the killer is.
3. It's a movie about a crazy person killing people.
お好みに合わせて使い分けてみてください。
ご参考にしていただければ幸いです。
ドイツ 
