Sorry but this is pretty nonsensical. First of all it shouldn't be this very far-fetched concept after we die according to atheism. We already have not existed for billions of years before we were born. It's just a matter of returning to the state of non existence we were in previously.

So if that whole concept of death is true then you were the only real being in the universe? I can kind of see what you're trying to convey but those are a poor choice of words. I think a better choice of words is that you're simply literally the main character. Which lots of people have already made that point before. There's a good Joe Rogan motivation clip that makes that point. But just because each person is the main character of their lives doesn't mean that everybody else isn't real.

But yeah it's kind of obvious that everybody should be the most important person for themselves. The typical exception is parent's love for their children. I will give you an interesting example of a thought i had that lines up with all of this quite perfectly. Back when i lived in Israel when i was younger i gave myself this thought experiment if i had to sacrifice myself to save the entire country of Israel and the answer i gave was that i wouldn't. It's because military is mandatory over there and that's why i was thinking about it. Because i so didn't care about protecting Israel i managed to get out of serving in the military though that's no easy feat. (had to convince them i was seriously suicidal)