Allah says is Quran in Sura Al-Ma’eda : If some one killed a person, not in punishment of killing or mischief in the earth, would be as killing all mankind, and he who saves a life is like saving the whole mankind. (Sura 5, verse 32).
Killing is one of the most dangerous major sins in Islam.
Killing can never be allowed but only as a punishment of crime of killing which should judged by an Islamic authority.
Wassalam.