There is no justification for killing any person be it a non-believer or someone who leaves their faith.
A person may choose to live or die for their faith. But there is no right in being murdered for leaving it, neither is their righteousness or justification.
God has not told anyone to murder another for their disbelief.
Murder is Murder there is no justification for such acts in religions which come from a true God.
Correct Murder is forbidden. There is no compulsion in religion, and people are free to come and go as they please, though to do so is making a mockery of God.
Indeed, those who have believed then disbelieved, then believed, then disbelieved, and then increased in disbelief - never will Allah forgive them, nor will He guide them to a way. Qur'an 4:137
And say, "The truth is from your Lord, so whoever wills - let him believe; and whoever wills - let him disbelieve." Qur'an 18:29
That's why one should study and be sure they are onto the truth before committing to any particular faith. If after doing that, you have doubts in the following days, weeks, months, ears etc, then fine go ahead and leave, keeping the matter between you, your family and God.
There shall be no compulsion in [acceptance of] the religion. The right course has become clear from the wrong. So whoever disbelieves in Taghut and believes in Allah has grasped the most trustworthy handhold with no break in it. And Allah is Hearing and Knowing. Qur'an 2:256
Under a Islamic State run with proper Shariah Law, (God's Law) people are free to leave their religion if they choose to do so.
In early Islam after the Prophet's death, there was a mass exodus of people who didn't really believe, and they sought to destroy Islam in its infancy. The decision was taken by the Caliph to fight these people and stem the exodus. Unique situation, unique time in History.
Elsewhere there were people who left freely, but then went over to the enemies of God and actively fought against the believers, either directly on the battlefield or indirectly by planting doubts in the minds of those weak in faith. Under Shariah Law this was classed as Treason, and such people could repent or refuse to do so and be killed.
Today Treason is still punishable by death even in some non Muslim Countries. America will kill its own citizens under Political Treason grounds without a trial or even arrest.
Anwar al-Awlaki - Wikipedia