Praise be to Allaah.
Your question may be answered by the following points:
(1) This is the ruling of Allaah and His Messenger, as the Prophet (peace and blessings of Allaah be upon him) said: "Whoever changes his religion, kill him." (reported by al-Bukhaari, al-Fath, no. 3017).
(2) The one who has known the religion which Allaah revealed, entered it and practised it, then rejected it, despised it and left it, is a person who does not deserve to live on the earth of Allaah and eat from the provision of Allaah.
(3) By leaving Islaam, the apostate opens the way for everyone who wants to leave the faith, thus spreading apostasy and encouraging it.
(4) The apostate is not to be killed without warning. Even though his crime is so great, he is given a last chance, a respite of three days in which to repent. If he repents, he will be left alone; if he does not repent, then he will be killed.
(5) If the punishment for murder and espionage (also known as high treason) is death, then what should be the punishment for the one who disbelieves in the Lord of mankind and despises and rejects His religion? Is espionage or shedding blood worse than leaving the religion of the Lord of mankind and rejecting it?
(6) None of those who bleat about personal freedom and freedom of belief would put up with a neighbour’s child hitting their child or justify this as "personal freedom," so how can they justify leaving the true religion and rejecting the sharee’ah which Allaah revealed to teach mankind about His unity and bring justice and fairness to all?
We ask Allaah for safety and health. May Allaah bless our Prophet Muhammad