Ethics help society function, agreed, but for a person who is entirely selfish, they would do all that they could thats unethical that wouldn't get them thrown into jail, or otherwise excluded. I also hold the notion that an extremely rational person would be self-absorbed, and selfish.
So what? So let that guy (let's call him Ebenezer) live his self-absorbed and selfish life. In your system, each of you will get your reward in the afterlife. In my system, the world I build for myself here is its own reward. Ebenezer has built his life for himself. Maybe it really sucks for him, maybe he is happy. Who are we to judge? If there is a god then that god will judge Ebenezer. I don't care. The beauty is that under either your "system" or mine, he gets what is coming to him.
I suppose I could be afraid that Ebenezer will be a total prick and somehow have a happy life anyway. Guess what. I will make myself (and possibly others) miserable looking for justice in this world by trying to make Ebenezer get what is "coming to him". This is where the Judeo-Christian view gets it screwed up. They spend so much effort trying to build God's kingdom here on Earth that they lose some of the joy of this fleeting existence.
Here is my recommendation for you humans: Enjoy this life and the gifts given to you by the Flying Spaghetti Monster (or whoever you worship) and try not to be an asshole while you are at it. That's where it is at. If you are doing wrong to your fellow man, he'll make you see the error of your ways pretty quickly. Make sure you can look yourself in the mirror at the end of each day and you will likely be okay with any just deity at the end of your all-too-short life.