Pondering  the almost-imponderable mental state of those who self-righteously take innocent human lives for this or that ‘higher’ cause — and, more generally, pondering the mental state of those who excuse all manner of offenses when done in the name of such causes — I recall something I read long ago in Thomas Babington Macaulay’s The History of England .
In this passage, Macaulay describes the Master of Stair (one John Dalrymple) who advised William III of England to order the Massacre of Glencoe  in 1692. (This massacre of a few dozen innocent Scottish highlanders was the result of William III’s attempt to subdue the Scottish highlands.) This atrocity is of no particular interest, but Macaulay’s description of Stair is. I write in bold the especially relevant insight:
The Master of Stair was one of the first men of his time, a jurist, a statesman, a fine scholar, an eloquent orator. His polished manners and lively conversation were the delight of aristocratical societies; and none who met him in such societies would have thought it possible that he could bear the chief part in any atrocious crime. His political principles were lax, yet not more lax than those of most Scotch politicians of that age. Cruelty had never been imputed to him. Those who most disliked him did him the justice to own that, where his schemes of policy were not concerned, he was a very goodnatured man. There is not the slightest reason to believe that he gained a single pound Scots by the act which has covered his name with infamy. He had no personal reason to wish the Glencoe men ill. There had been no feud between them and his family. His property lay in a district where their tartan was never seen. Yet he hated them with a hatred as fierce and implacable as if they had laid waste his fields, burned his mansion, murdered his child in the cradle.
To what cause are we to ascribe so strange an antipathy? This question perplexed the Master’s contemporaries; and any answer which may now be offered ought to be offered with diffidence. The most probable conjecture is that he was actuated by an inordinate, an unscrupulous, a remorseless zeal for what seemed to him to be the interest of the state. This explanation may startle those who have not considered how large a proportion of the blackest crimes recorded in history is to be ascribed to ill regulated public spirit. We daily see men do for their party, for their sect, for their country, for their favourite schemes of political and social reform, what they would not do to enrich or to avenge themselves. At a temptation directly addressed to our private cupidity or to our private animosity, whatever virtue we have takes the alarm. But, virtue itself may contribute to the fall of him who imagines that it is in his power, by violating some general rule of morality, to confer an important benefit on a church, on a commonwealth, on mankind. He silences the remonstrances of conscience, and hardens his heart against the most touching spectacles of misery, by repeating to himself that his intentions are pure, that his objects are noble, that he is doing a little evil for the sake of a great good. By degrees he comes altogether to forget the turpitude of the means in the excellence of the end, and at length perpetrates without one internal twinge acts which would shock a buccaneer. There is no reason to believe that Dominic would, for the best archbishopric in christendom, have incited ferocious marauders to plunder and slaughter a peaceful and industrious population, that Everard Digby would for a dukedom have blown a large assembly of people into the air, or that Robespierre would have murdered for hire one of the thousands whom he murdered from philanthropy.