and Allah says in the Qur’an:

وَلاَ تَقْتُلُوا النَّفْسَ الَّتِي حَرَّمَ اللَّهُ إِلاَّ بِالْحَقِّ وَمَنْ قُتِلَ مَظْلُومًا فَقَدْ جَعَلْنَا لِوَلِيِّهِ سُلْطَانًا فَلاَ يُسْرِفْ فِي الْقَتْلِ إِنَّهُ كَانَ مَنصُورًا

“Nor take life – which Allah has made sacred – except for just cause. And if anyone is slain wrongfully, We have given his heir authority (to demand Qisas or to forgive): but let him not exceed bounds in the matter of taking life; for he is helped (by the law)” (al-Isra’ 17:33)

According to the Qur’an, killing any person without a just cause is as big a sin as killing the whole humanity and saving the life of one person is as good deed as saving the whole humanity. (See al-Ma’idah 5:32)

However, your question is valid then how come the Qur’an says:

“kill them wherever you find them…”as it is mentioned in Surah al-Baqarah 2:191 and Surah al-Nisa’ 4:89. The answer is simple and that is you should read these verses in their textual and historical context. You should read the whole verse and it is better that you read few verses before and few after. Read the full text and see what is said:

وَقَاتِلُوا فِي سَبِيلِ اللَّهِ الَّذِينَ يُقَاتِلُونَكُمْ وَلاَ تَعْتَدُوا إِنَّ اللَّهَ لاَ يُحِبُّ الْمُعْتَدِينَ وَاقْتُلُوهُمْ حَيْثُ ثَقِفْتُمُوهُمْ وَأَخْرِجُوهُمْ مِنْ حَيْثُ أَخْرَجُوكُمْ وَالْفِتْنَةُ أَشَدُّ مِنْ الْقَتْلِ وَلاَ تُقَاتِلُوهُمْ عِنْدَ الْمَسْجِدِ الْحَرَامِ حَتَّى يُقَاتِلُوكُمْ فِيهِ فَإِنْ قَاتَلُوكُمْ فَاقْتُلُوهُمْ كَذَلِكَ جَزَاءُ الْكَافِرِينَ فَإِنْ انتَهَوْا فَإِنَّ اللَّهَ غَفُورٌ رَحِيمٌ وَقَاتِلُوهُمْ حَتَّى لاَ تَكُونَ فِتْنَةٌ وَيَكُونَ الدِّينُ لِلَّهِ فَإِنْ انتَهَوْا فَلاَ عُدْوَانَ إِلاَّ عَلَى الظَّالِمِينَ الشَّهْرُ الْحَرَامُ بِالشَّهْرِ الْحَرَامِ وَالْحُرُمَاتُ قِصَاصٌ فَمَنْ اعْتَدَى عَلَيْكُمْ فَاعْتَدُوا عَلَيْهِ بِمِثْلِ مَا اعْتَدَى عَلَيْكُمْ وَاتَّقُوا اللَّهَ وَاعْلَمُوا أَنَّ اللَّهَ مَعَ الْمُتَّقِينَ

“Fight in the cause of Allah those who fight you, but do not transgress limits; for Allah loves not transgressors. And kill them wherever ye catch them, and turn them out from where they have turned you out; for tumult and oppression are worse than slaughter; but fight them not at the Sacred Mosque, unless they (first) fight you there; but if they fight you, kill them. Such is the reward of those who reject faith. But if they cease, Allah is Oft-Forgiving, Most Merciful. And fight them on until there is no more tumult or oppression, and there prevail justice and faith in Allah; but if they cease, let there be no hostility except to those who practice oppression. The prohibited month, for the prohibited month, and so for all things prohibited, there is the law of equality. If then any one transgresses the prohibition against you, transgress ye likewise against him. But fear Allah, and know that Allah is with those who restrain themselves.” (al-Baqarah 2:190-194)