Salaam brother
That's a good question.
Leaving Islam in general is not the genuine of the problem as much as it is about propagation against it; because Islam is based on sincerity and Only God can put sincerity in the hearts of human beings. The main problem though is when the 'ex-Muslim' propagates against Islam and shows disrespect and hostility towards it.
Because I am not an official authority nor do I own anything of knowledge concerning this matter beyond merely 'my point of view' the website has regulations as to not give any answer regarding Islam with out evidence from at least one of the two main sources : Qur'an and Sunnah- Again I am no expert but the following are some verses from the Qur'an - Muslims dispute whether 'killing' the one who leaves Islam or not has any legitimacy though there is no particular verse in the Qur'an that says : 'Kill one who leave the religion' the Muslims who support this point of view base their Judgement one Hadeeth of the Prophet PBUH- (please pardon me for not recalling it),Others say it is not legitimate to kill anyone - even if they leave Islam.
From the Holy Qur'an:
1.256.
There is no compulsion in religion. Verily, the Right Path has become distinct from the wrong path. Whoever disbelieves in Tâghût- Misguidance and believes in Allâh, then he has grasped the most trustworthy handhold that will never break. And Allâh is All-Hearer, All-Knower.-[The Cow-The Holy Qur'an].
2.29.
And say: "The truth is from your Lord." Then whosoever wills, let him believe, and whosoever wills, let him disbelieve-[The Cave- The Holy Qur'an].
In the time of our Abubakr-RA he led the ' War against apostacy' but what were the causes of this war?
Is it because the people decided to leave Islam and join another religion? No. That wasn't the case,but the said we are Muslims but we refuse to pay the Zakat (Obligatory Charity), this is why Abu bakr gave them a chance and two but they still refused and it lead to the war because Abubakr did not want a new denomination forming - claiming that there is no Zakat in Islam and to protect the fundamental terms of Islam after the death of the prophet- the war was declared for the general benefit. In conclusion when it comes to something personal no one can interfere because it is between you and God; But when it concerns the General rights then there are some measure that are taken.
Musaylamah was a man who was a Muslim before but then; He claimed Prophecy at the time of the prophet PBUH, The Prophet only sent him a message telling him that -
The Land is God's He inherits it to whoever He wills; And the best ending is for the pious. The prophect did not order to kill him or any one who decided to follow him;but after the prophet's death
Musaylamah raged his followers to attack and kill the Muslims, It was obligatory for the Khalifah to declare war against him to ensure combat and self defense.
Only simple measurements, This answer can't be taken as 'the right' answer; I only wanted to give you an answer.
Hopefully you get better answers from other members.
Salaam.