They believe they have a right to do these things, and they do them as calmly as a butterfly takes to the air.
No cause is worth someone else's life, unless they choose to surrender it. No one is entitled to make martyrs of others. No reason makes sense of bombs and bullets or the killing of strangers and the murder of children. Nothing justifies such things, no grievance or crusade. And yet they are done.
All zealots are equal. All assassins are the same. No matter how different their beliefs, they share a common mind. Different ideologies merely mask what they have in common - arrogance, ruthlessness and self-righteous sickness.
The photographs of Anders Breivik on his way to court show a man whose eyes shine with a familiar deluded certainty. There is a calm disdain about him, a murderous smugness we've seen before. It is the look Hitler had, and bin Laden, and Stalin too. It is the strangely sane look of a mad man who has finally unleashed his demons and left his mark on the world.
We should not say that what Breivik did was inevitable. It wasn't. It is right to think such things will never happen. We should presume that. We need to have some level of trust in each other and we need to presume that our boundaries are, more or less, our neighbour's too or communities will not survive. They will be destroyed by a collective paranoia more corrosive than any vile act of individual madness.
Life is random enough without assuming every stranger we meet is a homicidal maniac, hell-bent on their own deranged agenda. However contentious something may be - and immigration has surely been a source of great conflict through all of human history - every citizen in an open society is right to assume the respectful values of tolerance and restraint will govern and constrain debate.
That's clearly what the people of Norway thought. Until one man proved them wrong. He did the unthinkable. He killed people because he decided they were traitors. And he saw no need for tolerance, respect or restraint. And no need for compassion.
Now he will stand trial. Third parties will repeat his grotesque rationalisations. Journalists will report them and nearly all of us will shake our heads in disbelief. But here's the next awful thing. Whatever his fate, however he is judged, whether he is imprisoned or found to be insane - surely for him the worst of all punishments - Mr Breivik has launched his crusade. He has spread his poison.
Like the butterfly, he's stirred the air. And, however much we may not wish it so, there will be some people, mad enough or bad enough to feel in the crazed chambers of their minds that he has shown them the way.
There always are. And always have been. The heroes of history have always had followers. But so have the villains. And there is always a deluded few who think the villains are heroes. If it were not so, the Nazis would have faded before their first rally, the IRA could not have waged its long war and al-Qaeda would have no converts now.
Now there is Breivik, the first of the new fanatics, although he will not be the last. There will be others, liberated by his rhetoric and his actions. Nothing happens in isolation. Like currents in the sky, deeds echo and resonate. We don't know with whom and we don't know when and we don't know where.
But we do know it will happen.