Praise be to Allaah.
Allaah has decreed what He has decreed. One of the pillars of faith is to believe in al-qadaa’ wa’l-qadar (the Divine will and decree). Allaah has created people and He knows what they are going to do, because of His vast knowledge. He has given His slaves freedom of will, and has shown us the right path, so whoever does good, it is for his own benefit, and whoever does evil, it will count against him. Allaah says (interpretation of the meaning);
“So whoever does good equal to the weight of an atom (or a small ant), shall see it.
And whoever does evil equal to the weight of an atom (or a small ant), shall see it.”
And Allaah says (interpretation of the meaning):
“By Nafs (Adam or a person or a soul), and Him Who perfected him in proportion;
Then He showed him what is wrong for him and what is right for him.
Indeed he succeeds who purifies his ownself (i.e. obeys and performs all that Allâh ordered…).
And indeed he fails who corrupts his ownself (i.e. disobeys what Allâh has ordered …).”
So it is not permissible for him to harm anyone unjustly, whether by killing or in any other way. Whoever does that deserves punishment in this world, and torment in the Hereafter, for going against the laws decreed by Allaah.