How would I explain to my children and students that it's OK to kill for revenge?

Since childhood, we are taught to respect and obey the law. The law is there to protect and serve us. We are led to believe that the law exists for the people by the people. Is that right?
In some developing countries, it is well known that the legal system protects constitutions rather than individuals. The citizens of these countries are mostly undereducated or undernourished, so they are not able to see or fight with the abusiveness of their governments or corporates. Those countries are recommended to look up to the advanced democracy of Europe or the USA. Certainly when I was growing up, I thought there was nothing European and American governments could do wrong. I thought they treated all their citizens and the citizens of other countries with respect. Sadly, the time and events showed me otherwise. Particularly witnessing USA and UK attack Iraq, left many people in shock and caused more hatred. I could not believe that to catch a terrorist group they would sacrifice so many innocent lives. After so many years of victims, suffering and pain, we heard that USA caught Osama in Pakistan. Yes, but why did they attack Iraq and Afghanistan without being exactly certain? No one can explain that clearly. There are rumours such as their will to control Middle East and the oil..
I honestly wanted to believe the opposite but then when the conflict in Yugoslavia started, America stood back. Women and children were begging for help. They claimed that they were being raped and murdered but none of the European and American governments moved a finger to help them. Some said this was because there was no petrol in Yugoslavia. I still wanted to see this as the governments not wanting to interfere with Yugoslavia's internal business. In the meantime, many innocent lives were lost.
I asked my children what they thought about America killing Osama Bin Laden. Their reply was that it was right and he deserved to die. How about law, justice, fairness, democracy and all of the above? I had to tell them that an unarmed man who did not resist to soldiers would not deserve to die even if this man was the most dangerous terrorist. What he would deserve is to be arrested and to stand trial before being brought to justice. Otherwise, this would be called murder. And what makes us better than a murderer, if we don't follow the law?