The life of a Muslim is sacred and can not be taken, with certain exceptions. Why certain Muslims kill each other can be explained in the following ways:
They do so in transgression. A person identifying as a Muslim does not imply that their actions are in accordance to Islam, rather they could be motivated by malicious intent or personal or ethnic disputes ...
The hadith you've mentioned is not at all quoted in any of the hadith compilation which are present on sunnah.com except with a hadith that starts similarly in Jami' at-Tirmdihi:
Narrated 'Abdullah bin Mas'ud:
that the Prophet (ﷺ) said: "A man among the inhabitants of Paradise will appear before you." So Abu Bakr appeared. Then he said: "A ...
The closest wording I could find in Musannaf Abd al-Razzaq is:
علقوا السوط حيث يراها أهل البيت
Hang your whip where it can be seen by the members of your household
— مصنف عبد الرزاق
There is dispute concerning its authenticy, some considered at least one of its chains to be Hasan while others consider them all to be Da'if. Refer to Islamweb:
The linked text says:
This mentions the grading of the Hadith based on Al-Albani's opinion, and gives cross-references to other books and the respective number of the hadith in each of them. The reason may be explained in one of those references. For example see it in hadith 342 in إرواء
The hadith is Sahih on the conditions of Bukhari and Muslim.
وهذا على شرط الصحيحين
ورجاله رجال الصحيح
Ibn Hibban: (also see Albani)
(Records it in his Sahih in كتاب السير).
حديث صحيح، محمد بن جعفر- وإن سمع من سعيد (وهو ابن أبي عروبة) بعد
الاختلاط - قد توبع، وبقية رجاله ثقات رجال الشيخين
The hadith you've mentioned can be found in Sahih al-Bukhari (see for example in the book of good manners and forms: al-Adab, in the book of asking permission, in the book of fighting for the cause of Allah (Jihad), in the book of Invocations and in the book of dealing with apostates) and in Sahih Muslim) in several chapters too. Imam al-Bukhari also ...
In Islam, no one is allowed to kill anyone whether he is a Muslim or no without a reason.
مِنْ أَجْلِ ذَٰلِكَ كَتَبْنَا عَلَىٰ بَنِي إِسْرَائِيلَ أَنَّهُ مَنْ قَتَلَ نَفْسًا بِغَيْرِ نَفْسٍ أَوْ فَسَادٍ فِي الْأَرْضِ فَكَأَنَّمَا قَتَلَ النَّاسَ جَمِيعًا وَمَنْ أَحْيَاهَا فَكَأَنَّمَا أَحْيَا النَّاسَ جَمِيعًا ۚ وَلَقَدْ جَاءَتْهُمْ رُسُلُنَا ...