Islam doesn’t call for killing all apostates, only some. It is to be decided by a judge, a person leaving soley because he doesn’t believe Islam is the truth isn’t killed, meaning an intellectual apostasy, rather the one who is killed is the one who apostates and rebels against the Islamic state. Or one who is discovered to have accepted Islam with the intention to leave it to cause doubt in the hearts of Muslims:
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A section of the People of the Book say: "Believe in the morning what is revealed to the believers, but reject it at the end of the day; perchance they may (themselves) Turn back;
If such a person is caught doing this, with evidence of course, such a person will be punished. . During the treaty of Hudabiya, the Prophet peace be upon him agreed with the non-Muslims of Mecca that any Muslim who apostates will be allowed to go to Mecca and live with them without any harm from the Muslims. He also agreed that any non-Muslim in Mecca who wants to become Muslim isn’t allowed to come to Medina. This shows that not all apostates are punished, it also shows that it doesn’t matter where a true believer is, he would never apostate.
Also I don’t know what you mean about denouncing Islam and Muhammad, if you mean disbelieving after belief, the same applies as I said above. However if you mean criticizing, then no. Muslims are open to criticism about their religion, however Muslims don’t allow anyone to mock their religion or anyone elses religion for that matter.
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Revile not ye those whom they call upon besides Allah, lest they out of spite revile Allah in their ignorance. Thus have We made alluring to each people its own doings. In the end will they return to their Lord, and We shall then tell them the truth of all that they did.