Hello and thank you for your very insightful questions. I am going to deal with your second question now, which hopefully will help explain the first.
You are correct in saying that to be a good person means following the principles of the Bible. In fact, your question was in a way asked by someone in the Bible, too. If you have time, read Mark 10:17-31 first before you continue reading this.
If you’ve just read the passage I referred to, you might be just as astonished as the disciples regarding Jesus’ answer! The rich young man was a good person. He had followed all the commandments. And yet, Jesus says he’s not saved! Why? Well, Jesus gave part of the answer earlier on, “no one is good—except God alone.” (Mark 10:18) Jesus continued to tell him that in order to be saved, the rich young man has to let go of what he holds dear to, and follow Jesus.
What Jesus is saying is that, yes, if we follow everything that God tells us to do—i.e. “to love God with all your heart and with all your soul and with all your mind and with all your strength” and to “love your neighbour as yourself” (Mark 12:30-31) —we will be saved. But Jesus knows—and if we are honest to ourselves, we will also know—that none of us are able to do this perfectly. It’s not enough to just try our best, or do it partially. If we don’t fulfill the commandments perfectly, we can’t be saved and are condemned as sinful.
In fact, the Bible tells us that no person on earth are able to fulfill God’s commandments perfectly, and so all are sinful.
Romans 3:10 “There is no one righteous, not even one ... there is no one who seeks God, all have turned away.”
Romans 3:20 “Therefore no one will be declared righteous in his sight by observing the law.”
It is because all of us are sinful, that God in his mercy has given us a new way to get saved—through believing in Jesus.
Romans 3:21-24 “But now a rightouesness from God, apart from law, has been made known…. this righteousness from God comes through faith in Jesus Christ to all who believe. There is no difference, for all have sinned and fall short of the glory of God, and are justified freely by his grace through the redemption that came by Christ Jesus.”
This is great news! Because by believing in Jesus, God freely forgives us our sins! Even if we’ve sinned a little bit (which to a perfect God, is still intolerably sinful) or if we’ve sinned a lot, if we believe that Jesus Christ died for our sins, and are willing to become followers of Jesus, we’re forgiven!
So going back to your scenario, well, unfortunately both man A and B would not be saved, unless one of them believes in Jesus and is willing to follow him as master of his life. To call Jesus as your master, you will actually try your best to follow the commandments, but you won’t ever be able to do it perfectly. And it doesn’t matter how well you do it—you’re not saved because of your “level of goodness”; rather you are saved because of your belief and trust in Jesus.
And Christians want to tell others about this forgiveness available in faith in Jesus, because we know that there is no other way to be saved, and we don’t want people to keep on trying to be good when that is not the way to eternal life. We tell people about Jesus because we love them and don’t want them to go to hell. There is no point encouraging people to just “be good” when that is a dead end for salvation!
As for your first question, there has already been a previous answer to the question of original sin in this site so because of space I won’t repeat it here. You can go to http://www.christianity.net.au/questions/from_christianitynetau_moblog911 to read it. If you still have further questions though, please feel free to ask again!