How can a human carry out a mass shooting whilst remaining calm?
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.