Quran 4:89 - But if they turn away, catch and slaughter them...

But if they turn away, catch them and slaughter them wherever ye find them; and (in any case) take no friends or helpers from their ranks." (4:89)

This verse has been misquoted like the previous verse, out of context. Here is the full passage:

فَمَا لَكُمْ فِي الْمُنَافِقِينَ فِئَتَيْنِ وَاللَّـهُ أَرْ‌كَسَهُم بِمَا كَسَبُوا ۚ أَتُرِ‌يدُونَ أَن تَهْدُوا مَنْ أَضَلَّ اللَّـهُ ۖ وَمَن يُضْلِلِ اللَّـهُ فَلَن تَجِدَ لَهُ سَبِيلًا ﴿٨٨﴾ وَدُّوا لَوْ تَكْفُرُ‌ونَ كَمَا كَفَرُ‌وا فَتَكُونُونَ سَوَاءً ۖ فَلَا تَتَّخِذُوا مِنْهُمْ أَوْلِيَاءَ حَتَّىٰ يُهَاجِرُ‌وا فِي سَبِيلِ اللَّـهِ ۚ فَإِن تَوَلَّوْا فَخُذُوهُمْ وَاقْتُلُوهُمْ حَيْثُ وَجَدتُّمُوهُمْ ۖ وَلَا تَتَّخِذُوا مِنْهُمْ وَلِيًّا وَلَا نَصِيرً‌ا ﴿٨٩﴾ إِلَّا الَّذِينَ يَصِلُونَ إِلَىٰ قَوْمٍ بَيْنَكُمْ وَبَيْنَهُم مِّيثَاقٌ أَوْ جَاءُوكُمْ حَصِرَ‌تْ صُدُورُ‌هُمْ أَن يُقَاتِلُوكُمْ أَوْ يُقَاتِلُوا قَوْمَهُمْ ۚ وَلَوْ شَاءَ اللَّـهُ لَسَلَّطَهُمْ عَلَيْكُمْ فَلَقَاتَلُوكُمْ ۚ فَإِنِ اعْتَزَلُوكُمْ فَلَمْ يُقَاتِلُوكُمْ وَأَلْقَوْا إِلَيْكُمُ السَّلَمَ فَمَا جَعَلَ اللَّـهُ لَكُمْ عَلَيْهِمْ سَبِيلًا ﴿٩٠﴾ سَتَجِدُونَ آخَرِ‌ينَ يُرِ‌يدُونَ أَن يَأْمَنُوكُمْ وَيَأْمَنُوا قَوْمَهُمْ كُلَّ مَا رُ‌دُّوا إِلَى الْفِتْنَةِ أُرْ‌كِسُوا فِيهَا ۚ فَإِن لَّمْ يَعْتَزِلُوكُمْ وَيُلْقُوا إِلَيْكُمُ السَّلَمَ وَيَكُفُّوا أَيْدِيَهُمْ فَخُذُوهُمْ وَاقْتُلُوهُمْ حَيْثُ ثَقِفْتُمُوهُمْ ۚ وَأُولَـٰئِكُمْ جَعَلْنَا لَكُمْ عَلَيْهِمْ سُلْطَانًا مُّبِينًا ﴿٩١﴾

4:88-91 Why should ye be divided into two parties about the Hypocrites? Allah hath upset them for their (evil) deeds. Would ye guide those whom Allah hath thrown out of the Way? For those whom Allah hath thrown out of the Way, never shalt thou find the Way. They but wish that ye should reject Faith, as they do, and thus be on the same footing (as they): so take not friends from their ranks until they forsake the domain of evil in the way of God (from what is forbidden). But if they revert to [open] enmity, seize them and slay them wherever ye find them; and (in any case) take no friends or helpers from their ranks. Except those who join a group between whom and you there is a treaty (Of peace), or those who approach you with hearts restraining them from fighting you as well as fighting their own people. If God had pleased, He could have given them power over you, and they would have fought you: therefore if they withdraw from you but fight you not, and (instead) send you (guarantees of) peace, then God hath opened no way for you (to war against them). Others you will find that wish to gain your confidence as well as that of their people: every time they are sent back to temptation, they succumb thereto; if they withdraw not from you nor give you (guarantees) of peace besides restraining their hands, seize them and slay them wherever ye get them; in their case We have provided you with a clear argument against them

So in the same manner as the first verse, this verse also only commands Muslims to fight those who practice oppression or persecution, or attack the Muslims. And in the event of a battle, the same laws of war are in place and a Muslim who transgresses limits should prepare for the punishment of God. In response to a question on verses 4:88-89, Dr. Muzammil H. Siddiqi quotes the verses in their full context and then asks the following:

Now tell me honestly, do these verses give a free permission to kill any one anywhere? These verses were revealed by God to Prophet Muhammad (peace and blessings be upon him), at the time when Muslims were attacked by the non-Muslims of Makkah on a regular basis. They were frightening the Muslim community of Madinah. One may say using the contemporary jargon that there were constant terrorist attacks on Madinah and in this situation Muslims were given permission to fight back the “terrorist”. These verses are not a permission for “terrorism” but they are a warning against the “terrorists.” But even in these warnings you can see how much restraint and care is emphasized. (SOURCE, emphasis added)

It is also important to note that the Qur'an clearly condemns murder. The Qur’an says about the prohibition of murder,

6:151 Take not life, which God hath made sacred, except by way of justice and law: thus does He command you, that ye may learn wisdom. 17:33 Nor take life, which God has made sacred, except for just cause. And if anyone is slain wrongfully, We have given his heir authority (to demand Qisas(retribution) or to forgive): but let him not exceed bounds in the matter of taking life; for he is helped (by the law) 5:32...if any one slew a person - unless it be for murder or for spreading mischief in the land - it would be as if he slew the whole people: and if any one saved a life, it would be as if he saved the life of the whole people

So the Qur'an makes it very clear that Muslims cannot initiate an attack against others, unless there is an immediate threat of being attacked. The context of the quoted verses applies only to situations where the oppressors are killing Muslims. In this case, they have a right to defend themselves and others, especially the weak and oppressed.