How should a Buddhist feel about the death of a person like Abu Musab al-Zarqawi? The worldly attitude is to say good riddance. This man organised the killing of large numbers of mostly civilian people in Iraq and now he too has been violently destroyed by an American bomb. Many promient people have expressed satisfaction.
Buddhists, however, have a different attitude because they have a different background theory about how the world works. Just as physicists wil ltell us that the destruction of the physical body by fire does not actually eliminate the matter nor the energy that, when conjoined in a certain pattern, constituted that body, so Buddhists believe that the moral force of a life is not destroyed by the mere physical death of the person who embodied that force. Karma is the equivalent in the moral domain of the law of conservation of energy in the physical one. Zarqawi's karma continues. Whether it will reassemble into a recognisable human personality package is an open question, but destroying the evil doer does not eliminate the evil from our world.
When the Buddha meets a mass murderer he does everything he can to convert him into a harmless one, just as he does with anybody else. Buddhas are not gods. They are inspiring and persuasive, but they do not create the universe nor can they miraculously change a person against that person's will. They seek rather to rouse a new gathering of the person's faith.
Zarqawi did not have much of a life. Starting off as a small time criminal he got swept to prominence by being caught by a big wave. That wave was the need of the USA to find some shred of credibility to support their completely false pretention that there was a direct link between Saddam Hussein and Osama bin Laden in order to shore up their case for the illegal invasion of Iraq. By naming Zarqawi as the link person (apparently without any evidence) at the United Nations, Colin Powell thrust Zarqawi into the limelight. Now, three years later they have killed him. This would all seem like a sick joke if so many other people had not also been killed along the way. As it is it is just to grievous for anybody to laugh.
The world will not be free of the karma of Zarqawi until all those who helped to make him into the symbol and the disaster that he was learn to be harmless ones and there is no immediate prospect of that. Zarqawi still walks this earth in the skins of all those who made him what he was. Some of them live in the Middle East and some in Western capitals. None of us is entirely innocent.