But how would he know he is moral? He’s just a guy that doesn’t kill people.
Adolf Hitler can make just as strong an argument for his own personal morality as can your hypothetical moral athiest. Belief in your own rightness is the only standard.
Okay, I'm going to clip it here, because the rest of this makes no sense. I'm also going to ignore the Hitler reference, because there is more than enough evidence as to where he got his morals from.
The problem you're having is that you're not understanding the concept of morals. All morals are subjective-even biblical ones. Take, for example, divorce. In the bible, Jesus says divorce is wrong-that it's immoral. However, even he allows that there is a time where it is okay to get a divorce-when your spouse commits adultery. So, divorce is wrong in some instances, but allowable in others.
Murder is the same thing, even biblically. One of the ten commandments says, plainly, "Do not commit murder." Therefore, murder is an immoral act. Again, though, the bible itself gives times when murder (defined as the taking of a human life) is allowable. Capital punishment, for example-numerous crimes in the Old Testament were considered punishable by death. In war, by definition, murder is allowed. Even people who kill in self defense are not considered immoral in the bible.
The difference between my morals (as an atheist) and your morals (as a Christian) is our source. You, as a Christian, get your morals from your god. I.e., a thing is good or bad because God tells us it is good or bad. As an atheist, I need to get my morals from my own thought process. For instance, here is how I came to the conclusion that murder is an immoral act:
I, by virtue of being human, have certain innate rights. I have the right to live, and I have the right to do what I need to do to continue living. Because you are also human, you also have this right. There is no objective reason why I could deny you this right. If I were to kill you (or any human), it would be denying that you had the right to live. Since there is no objective reason for me to do this, I consider murder to be immoral. Do I believe that there are situations where it would be alright to kill someone? Certainly. If my life, or someone else's was in jeopardy, and the only way I could save my life was to kill someone, I would consider it okay to do so.
Now, granted, my source of morals (the fact that I have value by virtue of being human, and, by extension, so do other humans) is not an objective source. It's a source that I came up with, and it works for me. Other people use the fact that they want to live in society and have access to all the benefits that society brings them, so it's important not to do things that would harm society in general, and the members of society specifically. It doesn't change the fact, however, that it is a source on which to base your morals. To me, it's a good source. It boils down to me treating others (as humans) the same way that I (as a human) wish to be treated.
