Can somebody explain to me how could some people become atheist. tell me whether what principles they believe in and why don't they believe in God.

I honestly dont know, but i think it might be that they dont want to believe in the fact that there is something "supernatural" or superior to everyone else.

Because that is what they believe in. There are multiple reasons for someone to believe in something.

I dont really agree about atheis, becouse if we dont have god, how we live and where we go after we death?

to answer the question more or less people who are believers to scientific facts that are being proven one by one ^_^

the believe god don't exist that might be the reason

People become Atheist for many reason.
Some are pushed too far for religion, some struggle and didn't get helped by religion.
But sometimes there's just not a reason. It seems like you're religious, and that's good for you. but some people don't believe easily. I'm personally an Athiest, but it doesn't mean I hate religion or dislike the feeling of believing in some supernatural. I think it's great that you can believe and that you have something. But the fact I don't believe doesn't mean that athiest live wrongly or have nothing to follow. We all still have morals that we follow. We just aren't doing it because of a book

I agree to some degree that it seems ridiculous to follow a book without proof to some atheists, but i'd also have to disagree. There is sufficient proof within the Bible if you look into it (although most of us still believe it mainly out of sheer faith). Look up the dead sea scrolls, Alexander throwing the rocks into the sea, and the theory of "intelligent design" if you're interested