Killing another Muslim
In the Mu`jam al-Awsat of al-Tabarani as mentioned by al-Haythami in his Majma` al-Zawa’id, 7/300 from `Abd Allah ibn Mas`ud that the Messenger of Allah said:
“A person who was killed will come on the Day of Judgment to the Almighty with blood gushing from his veins holding the arm of his killer saying: My Lord! Ask this person why he killed me! The Almighty will ask him: why have you killed this person? He will reply: I killed this man in order to make so and so person exalted and high. It will be said to him: this [right] is only for Allah and no-one else.”
يجيء المقتول آخذا قاتله وأوداجه تشخب دما عند ذي العزة فيقول يا رب سل هذا فيم قتلني فيقول فيم قتلته قال قتلته لتكون العزة لفلان قيل هي لله
The hadith is very weak but as identical corroboration in extended versions elsewhere (see: al-Nasa’i, Sunan [#4008] cf. al-Albani, Silsilat Ahadith al-Sahiha, #2698).
Killing another without just cause or reason is unlawful.
Killing for the sake of anyone else except Allah is unlawful.
Allah knows best.