Who was more conscious than the soldier of capricious fortune, of the random roll of the dice? - Robert Galbraith

He had an innate dislike of scheduled celebration and forced jollity, and of all inane practices, having ‘Happy Birthday’ sung to him was one of his least favourites.

There were undoubtedly those to whom killing was easy and pleasurable: he had met a few such. Millions had been successfully trained to end others' lives; he, Strike, was one of them. Humans killed opportunistically, for advantage and in defense, discovering in themselves the capacity for bloodshed when no alternative seemed possible; but there were also people who had drawn up short, even under the most intense pressure, unable to press their advantage, to seize the opportunity, to break the final and greatest taboo.

In essence, we tend to sort each other and ourselves into groupings, and that usually leads to an overestimation of similarities between members of a group, and an underestimation of the similarities between insiders and outsiders.