The NT has not abrogated the OT. Look at what Jesus said:
"Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. For truly, I say to you, until heaven and earth pass away, not an iota, not a dot, will pass from the Law until all is accomplished. Therefore whoever relaxes one of the least of these commandments and teaches others to do the same will be called least in the kingdom of heaven, but whoever does them and teaches them will be called great in the kingdom of heaven. For I tell you, unless your righteousness exceeds that of the scribes and Pharisees, you will never enter the kingdom of heaven." (Matthew 5:17-20)
The NT has just revealed the OT in a new way, exposing truths that were hidden from the creation of the world:
"For the law was given through Moses; grace and truth came through Jesus Christ." (John 1:17)
"Jesus spoke all these things to the crowd in parables; he did not say anything to them without using a parable. So was fulfilled what was spoken through the prophet: I will open my mouth in parables, I will utter things hidden since the creation of the world. (Matthew 13:34-36)
Moreover, the NT is the fulfillment of many of the prophecies contained in the OT.
Lots of Muslims have been killed for apostasy.
Ibn Abbaas said : The Messenger of Allah said, Whoever changes his (Islamic) religion, kill him. Al-Bukhary (number 6922)
Abd-Allah ibn Masood said : The Messenger of Allah said : It is not permissible to shed the blood of a Muslim who bears witness that there is no god except Allah and that I am the Messenger of Allah, except in one of three cases : a soul (in case of murder) ; a married person who commits adultery ; and one who leaves his religion and separates from the main body of Muslims. Sahih Al Bukhary number 6484 and Sahih Muslim number 1676
Ex-Muslims Testify on Islamic Intolerance in America: Apostates Denounce Islam, CAIR Council - YouTube