Why did Muhammad (SAW) say, "If anybody discards his religion then kill him"?

A Christian asked me that why Islam says to execute one who changes his religion. He gave a hadith as reference. The hadith : > Narrated Ikrima: Ali burnt some people and this news reached Ibn > `Abbas, who said, "Had I been in his place I would not have burnt > them, as the Prophet (ﷺ) said, 'Don't punish (anybody) with Allah's > Punishment.' No doubt, I would have killed them, for the Prophet (ﷺ) > said, 'If somebody (a Muslim) discards his religion, kill him.' " > > حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ، حَدَّثَنَا سُفْيَانُ، عَنْ > أَيُّوبَ، عَنْ عِكْرِمَةَ، أَنَّ عَلِيًّا ـ رضى الله عنه ـ حَرَّقَ > قَوْمًا، فَبَلَغَ ابْنَ عَبَّاسٍ فَقَالَ لَوْ كُنْتُ أَنَا لَمْ > أُحَرِّقْهُمْ، لأَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ ‏"‏ لاَ > تُعَذِّبُوا بِعَذَابِ اللَّهِ ‏"‏‏.‏ وَلَقَتَلْتُهُمْ كَمَا قَالَ > النَّبِيُّ صلى الله عليه وسلم ‏"‏ مَنْ بَدَّلَ دِينَهُ فَاقْتُلُوهُ > ‏"‏‏.‏ > > Reference : Sahih al-Bukhari 3017 > In-book reference : Book 56, Hadith 226 > USC-MSA web (English) reference : Vol. 4, Book 52, Hadith 260 So what should be my reply ? What is the explanation and the context of the hadith? What is the logical answer? We know that in Islam, punishment behind other crimes such as fornication, orders to kill mushrik in Quran etc. has a background, context and logic. So what is the logic, background and context behind this ? It will be helpful if anyone answer.