Hadith: The most rebellious of people against Allah, Exalted and Glorified, is someone who kills another in the sacred precincts of Allah

عن عمرو بن شعيب، عن أبيه، عن جده، قال: قال رسول الله صلى الله عليه وسلم :"إن أعتى الناس على الله ثلاثة: من قَتَلَ فِي حَرَمِ اللهِ، أَوْ قَتَلَ غَيْرَ قَاتِلِهِ، أَوْ قَتَلَ بِذُحُولِ الجاهلية".
[حسن] - [رواه ابن حبان وأحمد، وأصله في البخاري من حديث ابن عباس]

‘Amr ibn Shu‘ayb reported from his father, who reported from his grandfather: The most rebellious of people against Allah, Exalted and Glorified, is someone who kills another in the sacred precincts of Allah; kills someone other than his killer; or someone kills someone out of a pre-Islamic animosity.
Hasan/Sound. - [Ibn Hibbaan]

Explanation

‘Abdullāh ibn ‘Amr (may Allah be pleased with him) said that the Prophet (may Allah's peace and blessings be upon him) had said that the most rebellious and tyrannical in the sight of Allah, as far as murder is concerned, are three people:
1. Someone who kills someone else whose blood is inviolable under the Shariah in the safe sanctuary of Allah; Makkah. A forbidden killing is the gravest sin after polytheism, and it is more forbidden and sinful when it is committed in the sanctuary of Allah. In this respect, Allah, the Almighty, says: {Whoever intends [a deed] therein of deviation [in religion] or wrongdoing - We will make him taste of a painful punishment} [Sūrat al-Hajj: 25]. In an authentic Hadīth, the Prophet (may Allah's peace and blessings be upon him) said: ''Your blood, honor, and properties are as inviolable to you as this month of yours in this country of yours.'' [Muslim: 1218]
2. Someone who kills someone other than his killer (the person who committed the murder). Allah, the Almighty, says: {No bearer of burdens can bear the burden of another} [Sūrat al-An‘ām: 164]. That is to kill someone other than his killer or to kill, along with his killer, someone else. Excessive murder was a pre-Islamic tradition, that Allah has forbidden.
3. Someone who kills due to the pre-Islamic animosities and feuds that Islam revoked. Exceptions, however, are someone who kills another while defending himself and not for any other reason. Also, killing someone who commits a crime that legalizes killing him, such as deliberate murder in the sanctuary, so the sanctuary does not provide a safe place for people to commit crimes.