So in other words, you did not choose your beliefs.
Please explain why do you believe so?
I believe that murder is wrong, like in the case of the death penalty,
Actually, murder is unlawful killing. Death penalty is usually based on law, and therefore not unlawful and not murder. Still, you could think it is wrong, but there is really difference between those words.
and yet, you're saying that it's not useful for me to believe in the existence of God. Why is that? According to you, understanding what's right and why it's good, is the reason why it should be useful for me.
I think you have not understood what I say. Understanding what is right and good doesn’t really depend on do you believe in His existence. That is why believing in His existence is not crucial.
Explain to me why it's right to kill another person, lie and steal in one particular situation, but it's wrong to do the same in a different situation?
I don’t think lying and stealing are right in any case. And killing, I accept, if God kills, because He has given life and therefore has also right to decide how long life He gives. There is really no obligation why He must give more than what He likes to give. And by what the Bible tells, God gives eternal life only for righteous. I think it is good, because the unrighteous ones would make life eternal suffering, if they could live forever.
Explain why it's wrong to commit a global genocide of the vast majority of the Earth's population, or why it's wrong to punish a person because his ancestors/forefathers did something that was wrong, or why it's wrong for a human being to own another human being as property?
People have no right for any of those, therefore it is wrong, if people would do those.