I agree with Caz. If these hijackers had been genuinely good people their minds could never have been twisted by religion. They took the worst aspects of the Koran, in the same way as the Crusaders took the worst aspects of the Bible, and used it to support their evil deeds whilst comforting themselves with the thought that they were carrying them out in the name of their 'god', so the seeds of evil must have been there in the first place. I think it's long been accepted that some people are born evil, otherwise how do you explain psychopaths, for example?
Also, psychopaths are not evil. What they lack is a theory of mind, empathy, etc. They essentially have a damaged brain.
Let's take someone you're close to, for example. Imagine they are in an accident and suffer a brain injury that fundamentally changes them. They no longer feel any kind of empathy and they can't relate to other human beings. For all intents and purposes, they are now a psychopath. Do you now view them as evil?
There are a large number of psychopaths in the general population, most of whom do not turn out to be killers.