''The horror, the horror.'' So wrote Joseph Conrad in his African novella Heart of Darkness.
The words are easily recalled when confronted with details of the brutal murder of a man - reportedly a soldier - in a London street early yesterday (New Zealand time).
The attack, in which two young men rammed the victim with a car and then hacked him to death with cleavers, occurred in the afternoon on a Woolwich street in front of dozens of pedestrians and motorists.
The assailants were subsequently shot and wounded by police and taken into custody. Before police arrived, one of the men was recorded on film saying the attack was carried out in retaliation for the British Government's military involvement in Arabic countries, that it was ''an eye for an eye, a tooth for a tooth'', and ''we swear by almighty Allah we will never stop fighting you unless you leave us alone''.
The images came just days after a video posted on the internet apparently showed a rebel commander in Syria cutting out and biting into the heart of a dead government soldier, one of an increasing number of such videos accompanied by commentary about revenge for the atrocities allegedly committed by Syrian President Bashar al-Assad's regime.
The brutality of such acts is incomprehensible. Horrifying. Sickening. But, sadly, they have become a way of life in war zones around the world. If we are lucky enough to have grown up in a country in which such terrors are not commonplace, we may feel despair, hopelessness and helplessness, and also an uneasy gratitude it is not our reality.
Part of the horror of the grotesque London act was it occurred in daylight, out of the blue, on a member of the British armed forces, in a British street, in front of British passersby going about their daily lives.
Mass attacks have maximum impact for terrorists, but people can get lost in the statistics. An attack such as this is personal. It is designed to provoke shock, dread, intimidation and fear. It could be any one of us, at any place, at any time. No one person is safe. That is the message.
Heart of Darkness explored issues of civilisation versus savagery; racism, imperialism and the abuse of power; and the conflict between good and evil.
The ''horror'' Mr Kurtz exclaims in his dying moments referred not just to what he had witnessed, been responsible for, and become, but could also be seen as the wider horror of what humanity had become.
The backdrop to Conrad's novel was Africa under colonial rule; the same themes are prevalent more than 100 years later in the Western military intervention in the Middle East.
There will be many questions in the wake of this tragedy. They will include whether the authorities could have prevented it, soul-searching about foreign military intervention and its role in fundamentalism, and comments about immigration. Another will be how can we stop it happening again?Sadly, we can't.
We can't identify and predict all the people who might one day commit a violent offence - for whatever reason. We certainly cannot stop people determined to kill in the name of their gods.
But if ''the only thing necessary for the triumph of evil is for good men to do nothing'', then we must all do something. We must make our own actions about love, not hate; life, not death; courage, not fear; understanding, not ignorance; acceptance, not intolerance; forgiveness, not revenge. And we can pass those values on to others.
One ''good woman'', 48-year-old mother-of-two and cub leader Ingrid Loyau-Kennett, showed amazing courage yesterday, reportedly calmly confronting the attackers. Another woman reportedly comforted the victim. Both did what they did at considerable risk to themselves.
They might not have been able to save the victim. It might have been too late for him to hear words of comfort, or feel loving human touch. But others saw what happened, his family and friends will know, and the world will read and hear of their actions.
Those are the actions we can choose to remember, and the message we can choose to hear. It is so simple, anyone can do it. When faced with darkness, seek the light.
