Why God Casts Terror into the Hearts of the Unbelievers?

Atheist: God says that he will cast terror into the hearts of the unbelievers and send them to hell because they associate partners with him without authority (The Quran 3:151).

Muslim: So?

Atheist: God is so mean.

Muslim: If you do something that you are not authorized to do, you will be punished. But if you do something that you are authorized to do, you will not be punished. This sounds rational, not cruel.

Atheist: Who gave God authority to decide what I am authorized to do and what I am not?

Muslim: If you are the owner of something, you have authority over that thing. Again, this sounds very rational to me. God is the owner of this universe and everything in it including human beings. God says in the Quran – “O company of jinn and mankind, if you are able to pass beyond the regions of the heavens and the earth, then pass. You will not pass except by authority from God.” (The Quran 55:33)