Why is a person who becomes a non-Muslim after being a Muslim condemned to death in Islam?

Why is a person who becomes a non-Muslim after being a Muslim condemned to death in Islam? If such a verdict is not truly Islamic why do the Muslims believe in it?

Answer

Let’s distinguish between Islam — as mentioned in the Quran and the sunnah practice of the prophet — and Muslims, the alleged followers of the message. If it is shown convincingly that Islam has a certain rule, or doesn’t have one, it should be a good enough reason for a rational individual to be convinced that what the religion says is not what its followers are doing.
The Quran has very clearly mentioned that the immediate addressees of the messengers (rasul, rusul) had to face death if they were not to accept the message of the messengers communicated to them directly. That’s why, for instance, the nation of Noah, alaihisaalaam, was drowned after they had rejected his message. A simliar fate awaited the nation of prophet Muhammad, alaihissalaam. The rule has nothing to with the rest of the people. The Quran is unmistakbly clear about this matter.
It was on the basis of this rule that it had to be declared that the same very people who had to believe in the message of the prophet to survive to begin with, if they were to renounce their faith, they again had to face death. That is what it means in the hadith of the prophet which says “If they turn their backs on their faith, kill them.”
It would be ridiculous, immoral, and self-contradictory approach to assume that Muslims can convert non-Muslims to Islam, but if a fellow Muslim converts to another religion, he/she has to be beheaded. For God’s sake, this not Islam. I hope I have clarified.