He seems to have as a basic assumption that people who think people are born basically good, can't also think that they can go bad. He seems to limited his thinking to subset of the human population.

First, if you believe people are born good, you will attribute evil to forces outside the individual.

Not true, if don't believe in a supernatural being who created the world then it is hard to attribute evil to an outside force.

Second, if you believe people are born good, you will not stress character development when you raise children.

Absolutely wrong, and it should be obvious that it is wrong. Not everyone, religious or not, makes it point to teach their children how to make the world a better place. But, stressing character development and overcoming the human condition is limited to people who think liek he does.

Third, if you believe that people are basically good, God and religion are morally unnecessary, even harmful.

He seems to unfamiliar with the diversity of opinion in the Christian church, let alone others outside the 3 major reveled religions.

Fourth, if you believe people are basically good, you, of course, believe that you are good -- and therefore those who disagree with you must be bad, not merely wrong.

Wrong. In my experience this kind of thinking is more prevalent in people who in absolutes, regardless of what their religious beliefs, or lack thereof, are.

Understanding that others can be good or bad, and that even good people commit evils acts is not limited to people who reject the notion that people are basically good. There are many varying viewpoints about how and why people commit evil acts and he is over vastly simplifying that discussion.

His views to me are not abhorrent, the are simplistic. He seems to be lumping people into two groups. One group who believe that man has to struggle to overcome his bad nature. Otherwise, you believe the external forces cause people to do bad and evil things. Where do you put the people who believe that some people are born with the capability to commit evil acts and others develop it later in life?

In my opinion, you don't have to have an absolute view of morality to understand absolutism or even be effective dealing with people who think that way.

Even though you can glean from his writings the point about the absolute values of the religious terrorists we are currently fighting, other writers have made that point more directly.