Al Quran: And whoever kills a believer
intentionally, his recompense is Hell to abide therein; and the Wrath and the Curse of ALLAH are upon him, and a great punishment is prepared for him.
Al Quran Surah An Nisa (4) :93
Al Quran: If anyone killed a person not in retaliation of murder, or (and) to spread mischief in the land - it would be as if he killed all mankind, and if anyone saved a life, it would be as if he saved the life of all mankind.
Al Quran: Surah Al-Maeda (5) Verse 32
Hadith: Narrated 'Abdullah bin 'Umar Radhi Allahu Anhuma : The Prophet Sal-Allahu Alaihi Wasallam said, "After me do not become disbelievers, by striking (cutting) the necks of one another.
Sahih Bukhari, Volume 9, Book 83, Number 7:
Narrated 'Abdullah bin masood Radiallahu anhu : The Prophet Sal-Allahu Alaihi Wasallam said, "Abusing a Muslim is Fusuq (an evil doing) and killing him is Kufr (disbelief)." Sahih Bukhari, Vol 1, Book 2, No. 47: