I am a student of law and presently working on the comparative study of murder and homicide law in Islam and in the Western legal tradition. While doing research on the topic of honour killings I came across with the various judgments of Pakistani courts and of other Islamic countries courts wherein the judges viewed such killings as justified for various reasons:
1. the adulterous is otherwise liable to be punished with death,
2. the murder is committed because of ghairat and the prophet (pbh) has asked people to defend their ghariat,
3. murder is committed because of IZTARAR
4. the person who commits adultery becomes ghair Masoom-ud-dam
5. the killing is done in the discharge of duty which rests upon every Muslim to eliminate evil in possible ways.
These are the five main contentions with some more flimsy reason/grounds they consider while dealing with such murderers with indulgence. I have some very serious objections on all of their arguments. Firstly, Nobody in Islam is allowed to take law in his own hands. Secondly, if the people committing adultery are murdered how the murderer would prove that they were committing such an act which was liable for the death penalty. Thirdly, in saying this he should have brought the evidence which is required to prove the offence of zina - four witnesses who had seen the commission of the sexual acts. Fourthly, no individual can declare another individual a gair Masum-ud-dam. Fifthly, the obligation to eliminate the evil cannot be extended to an extent wherein the wrongdoer may be killed.
There is a lot which my research suggests but I would like to take benefit of your opinion before concluding my research on this topic. I shall be very grateful if you could explain the issue with reference to some primary sources of Islamic law.