"Who allowed them to kill a human being?"
The scholars, the mullas and the people who believe in fictitious hadith and have a distorted understanding of the Quran.
Where does the Quran say you can kill someone for blasphemy, insulting another, or rejecting Allah? It does not. Yet when these incidents occur a backward scholar will quote Quranic verses out of context, add one line here to another there, and present fairy tale hadith to justify this barbarity, deceiving the masses that it is 'islamic', a 'sunnah' so is justified!
When 'believers' are deaf, dumb and blind to reason, then anything unreasonable goes, no matter how sickening the act.