How can a human carry out a mass shooting whilst remaining calm? In the wake of the horrific terrorist attack in Christchurch New Zealand, it became known that the shooter had live streamed 17 minutes of his rampage to Facebook. In the video, of which I have only watched about 15 seconds from the point of the first shot being fired, the shooter can be seen extremely calmly carrying out his attack.

My question to the people here is, what is the root cause of a human being such as the shooter, being able to carry out such inhumane, disgusting, horrifying and downright evil acts whilst remaining so chillingly calm?

As someone with very little to no knowledge or experience with the relevant psychological literature, I have to ask, is this shooter a straight up psychopath? Surely the people who carry out such attacks have had some prior brain damage or genetic mental condition?
There may be less apparent contradiction if the shooter doesn't consider those behaviors as immoral, disgusting, etc. Consider a soldier in wartime: they might sometimes appear calm as well.

If you look at the stated justifications for mass shootings, they often have a specific purpose and apparently see moral reasons for their actions. Broadly speaking, people differ widely in what they consider ethical or moral. One person's virtue is another's sin. There are shared features between many ethical traditions, such as a prohibition against murder, but murder and war remain common. There is no need for a specific physiological fault like a brain injury or a particular genetic variant for humans to disagree wildly on what actions are appropriate. It's famously difficult to sympathize with the moral intuitions of other people; see for example https://en.wikipedia.org/wiki/Moral_foundations_theory. Instead of universal reasons, we generally have the strong feeling that something is right or wrong, and then we construct reasons.