How should we understand the command of Islam ‘Kill the one who deserts Islam’?

How should we understand the command of Islam ‘Kill the one who deserts Islam’? Please tell me where I can find these answers in the Quran, the Hadiths, the Islamic laws, etc. Thank you

Dear Brother / Sister,

Man of tabligh (conveying the message of Islam) is anguished; he becomes very sorrowful when people go astray and disobey Allah by transgressing His commands. Conversion from Islam distresses him terribly; when he becomes desperate in the name of tabligh, he becomes infuriated and suffers palpitations. When, in the Quran, our Prophet is addressed as: "It may be thou frettest thy soul with grief, that they do not become Believers"(ash-Shuara, 26/3), the anguish of the Messenger of Allah and his mood originating from that anguish is described. As a matter of fact, that mood is and should be present in each man of tabligh.
Conversion (irtidad) means deserting the religion. The murtad is the person who denies all of the holy things that he had believed before. That person is regarded to have betrayed Islam, in a sense. A person who betrays once can always betray. Therefore, according to some scholars the murtad has no right to live. However, according to the form our fiqh scholars systematized, first, the murtad is informed about the issue that caused his conversion in detail. He is followed and observed for a while and he is tried to be persuaded about the issues that he feels doubtful. If none of those efforts turn out to be fruitless and it becomes clear that that person is a malignant tumor, a source of trouble, his punishment is executed. [1> However, no believer can be indifferent to the conversion of someone from Islam. The concept of humanism of Islam is contrary to it. Every believer who hears about the case feels sorry and anguished about the conversion. However, the anguish of man of tabligh is deeper than that of everybody else because the guidance of people is the aim of his existence.
Here is the mood of the Messenger of Allah (pbuh) when he hears about what Kalid b. Walid (may Allah be pleased with him) has done. Hazrat Khalid carries out an execution by hurrying about the principles of the religion regarding conversion. When the Messenger of Allah (pbuh) hears it, he becomes very sorry; he prays to God Almighty by raising his hands and says: "O Allah! I take refuge in you from what Khalid has done.[2>
That sensitivity of the Messenger of Allah (pbuh) is seen in the same form in the people around him. For instance, Hazrat Umar (may Allah be pleased with him) asks someone coming from Yamama whether there was anything serious there or not. The man says there is nothing serious or important except someone who has converted from Islam. Hazrat Umar (may Allah be pleased with him) stands up excitedly and asks, “What did you do to him?” When the man says,”we killed him”, Hazrat Umar (may Allah be pleased with him) sighs just as the Messenger of Allah (pbuh) did and says to the man, "Should you have not imprisoned him and waited for a while? Then, he raises his hands and prays to Allah as follows: "O Allah! I swear that I was not there when those people did it. I also swear that I did not like what they did when I heard about it."[3>
[1> Bukhari, Diyat, 6; Muslim, Qasama, 25; Sarahsi, Mabsut, 10/98; Kasani, Badiu's-Sanai, 7/134.
[2> Bukhari, Maghazi, 58; Ibn Hisham, Sirah, 4/72.
[3> Muwatta, Aqdiya, 58.

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