The importance of survival and continuation of mankind's life in the Islamic law is so much that in order to safeguard it the law of retaliation has been legislated as a guarantee for mankind to survive.
The philosophy of legislation of the law of retaliation and Dieh (blood money) in Islamic law is to safeguard man before unlawful violation and intrusion of individuals upon life and wellbeing of others to prevent bloodshed in a society.
وَلَكُمْ فِي الْقِصَاصِ حَيَاةٌ يَا أُولِي الْأَلْبَابِ لَعَلَّكُمْ تَتَّقُونَ
And there is for you in legal retribution [saving of] life, O you [people] of understanding, that you may become righteous. (Quran, 2:179)
This law prior to advent of Islam has also been ruled in the Torah (the Holy book of Judaism). Then it was expressed by Islamic law in a more complete form.
وَكَتَبْنَا عَلَيْهِمْ فِيهَا أَنَّ النَّفْسَ بِالنَّفْسِ وَالْعَيْنَ بِالْعَيْنِ وَالْأَنْفَ بِالْأَنْفِ وَالْأُذُنَ بِالْأُذُنِ وَالسِّنَّ بِالسِّنِّ وَالْجُرُوحَ قِصَاصٌ ۚ فَمَنْ تَصَدَّقَ بِهِ فَهُوَ كَفَّارَةٌ لَهُ ۚ وَمَنْ لَمْ يَحْكُمْ بِمَا أَنْزَلَ اللَّهُ فَأُولَٰئِكَ هُمُ الظَّالِمُونَ
And we ordained therein for them: "Life for life, eye for eye, nose for nose, ear for ear, tooth for tooth, and wounds equal for equal." But if anyone remits the retaliation by way of charity, it shall be for him expiation. (Quran, 5:45)
But in the eyes of Islam, retaliation never means vengeance or equal retaliation to shed blood against bloodshed, but deciding a proportionate punishment so that (other) people will be considerate about the sanctity of each other's lives and to avoid murder. If you pay attention to the cases of retaliation and its philosophy (for existence) then any skepticism, doubt or suspicion about this advanced Islamic decree will be erased from your mind.
Execution of a retaliation decree In Islam is dependent on a few things:
• If the killer was in a sound mental health condition
• If it was deliberate
• The victim's immediate family won't agree to replace the actual punishment with money or pardon of the killer.
Otherwise retaliation decree for the killer who due to insanity or mental disorder or if deprivation of life is not deliberate due to an error or any condition in which the killer can be subject to pardon or substitution with financial payment as a reimbursement, then a retaliation decree will not be executed.
وَمَا كَانَ لِمُؤْمِنٍ أَنْ يَقْتُلَ مُؤْمِنًا إِلَّا خَطَأً ۚ وَمَنْ قَتَلَ مُؤْمِنًا خَطَأً فَتَحْرِيرُ رَقَبَةٍ مُؤْمِنَةٍ وَدِيَةٌ مُسَلَّمَةٌ إِلَىٰ أَهْلِهِ إِلَّا أَنْ يَصَّدَّقُوا
And never is it for a believer to kill a believer except by mistake. And whoever kills a believer by mistake - then the freeing of a believing slave and a compensation payment presented to the deceased's family [is required] unless they give [up their right as] charity. (Quran 4:92)
When a retaliation decree is executed, individuals will be less likely to commit murder and sanctity of mankind will be safeguarded.