The answer is all too easily if someone can convince them that it is for the good of the cause. Ask anyone to torture an aid worker for weeks and then kill them by hand in the most painful and brutal way and ordinarily they won't be able to do it. Get that person to believe that they belong to a special group of people and the cause needs them to sacrifice their natural feelings and prove their dedication and they become all too capable of doing exactly that.
Religious belief can be one of the most effective ways of doing this. From the earliest times people have proved all too capable of murdering their neighbours because they followed different religious leaders. It is hard to manipulate someone whose religion has taught them to distrust leaders and about the importance of tolerance, understanding and making your own moral decisions. It is much easier to manipulate someone when their religion tells them that there can only be one interpreter of the truth and they need to rote learn that ideology.
But it isn't just authoritarian religion that has the power to turn us into killers for the cause. In Rwanda people were prepared to do it because of tribal loyalty. In Communist Russia and Communist China people were prepared to denounce their neighbours to the secret service knowing that other people would torture them before sending them to camps to be worked to death. Hundreds of thousands of people loyally turned up to work every day to run the camps and prisons in which millions died. These killers thought they were helping to build socialism. It made it easier to pull the trigger in the depths of one of Beria's interrogation centres. Even the racist guards at Auschwitz thought they were helping humanity. They were told they were part of an elite tasked with the necessary work of cleaning it up. The Nazis convinced each other that industrially organised mass murder was necessary and that their death camp workers were particularly brave heroes in a noble cause.
Once someone has entered a cult mentality they can easily be persuaded that those who inflict the greatest of cruelties are the most admirable and dedicated whilst anyone who behaves humanely is suspicious and weak. Once someone has adopted this kind of narrow minded commitment to a cause to the exclusion of everything else there is very little that can be done to change their minds. Logical argument or kind and reasonable behaviour are unlikely to change them as they will be seen as a trick. In these circumstances there are times when the only option is to use violence. When you are dealing with an out and out fascist who is out to kill you then you are entitled to use violence to defend yourself and your opportunity to live by your own values. That is why I am not a pacifist.
But before we enter any war, including a war against terror it is really important a great deal of care is taken to ensure that what we are doing is wise and helpful and that we are not doing more harm than good. History is littered with examples of bad wars that began because people had a thoroughly legitimate moral outrage to defend. So when it looks like the use of violence would be entirely legitimate it is important to pause for a while and ask another really important question. Will it actually help or will it do more harm than good on this occasion?
It is that test that I think most of the politicians in the West have been failing. I have no difficulty in accepting that we should fight back against the blind loyalty to a horrible ideology adopted by members of ISIS. My problem is in believing that what our politicians are proposing to do will make anything better. Indeed I think they will make things a lot worse.
Consider for example the pros and cons of bombing ISIS in Syria. It is entirely possible that the UK's air force could fly over Syria and hit some targets that would harm the military capability of ISIS. But think for a minute how it would feel if you were in Reading and a Syrian bomber came over and killed one of the locals. Even if you thought that the person killed was a nasty piece of work who had it coming how would you feel about the nation that sent the aircraft? How would you feel if the person killed was your brother, your friend, your neighbour? And if only one of the air strikes missed its target selected on the basis of unreliable local intelligence reports and a school was hit what would your attitude be then?
UK bombs in Syria are certainly of doing significant damage to existing terrorists. But they risk recruiting a lot more terrorists than we destroy and therefore making things worse. Some of those newly recruited terrorists will live in the UK and be completely off the radar of any security service.
What is worse is that bombing cannot possibly win the war. That needs a ground campaign. That cannot possibly succeed work without a proper UN agreement on who should run the country and more importantly without that agreement standing a realistic chance of gaining majority support across all sections of the local population. It might take a great deal of difficult, painstaking and frustrating work to reach an agreement with the Russians but that work needs doing not ignoring. A huge and sustained genuinely international commitment to restore peace in Syria could conceivably work. One more nation dropping bombs without being clear about what will happen next couldn't. A reasonable case can be made for a UN organised settlement backed by multinational military action. Until that happens the key principle should be "First do no harm". The UK is better off staying out of conflicts that are beyond its ability to control or influence positively.
Recent experience shows that it was reasonably easy for the coalition forces to occupy Iraq. Bush was able to declare that the war was won within weeks of it starting. He was wrong. Does anyone now think things are better there than before? ISIS occupies territory that British soldiers died for and uses captured weapons that British taxpayers paid for. The invasions caused more chaos than it cured. Bombing Libya hasn't exactly created a hotbed of liberty. It might just be possible to make an argument that the invasion of Afghanistan has made some parts of the country much pleasanter to live in than before. It made others worse and it is far from clear that Afghanistan as a whole has benefitted from the invasion. The price paid in terms of the lives lost is all too clear.
So until we have a really clear strategy that has genuinely broad based international support and stands a good chance of being popular on the ground it is better to do nothing than to help create even more misery. If we really feel the need to do something urgent to help halt ISIS then there is one hugely practical step that we could take.
Tunisia has stood out bravely against the trend towards narrow minded extremist interpretations of Islam and have been attacked by terrorists for its trouble. As a result Tunisia has lost tourist business and is in severe danger of tipping into an economic crisis that could destabilise the country and play straight into the hands of ISIS. The easiest, quickest and most effective way to make a positive difference in the battle against ISIS is to help countries that country to remain stable and prosperous. Sending serious quantities of aid and support to Tunisia would help to create a beacon of stability and success that ISIS would hate. Dropping bombs on Syria for no clear purpose would help them enormously. Which do you think is being most effectively tough on terrorism and the causes of terrorism?