The thing is, there are many rulings in islam that are there but are not implemented in most parts of the world because there is no islamic shariah system of law enforcement. Only a very few parts of the world implement those laws. And there where does laws are implemented, apostates are killed. Like in Saudi Arabia.

And I have no idea what you are talking about? Not being allowed to kill another man unless you are free of sin?

The ruling regarding apostasy or any other issue that requires an Islamic court of law to sanction the death penalty is supported either by qur'an or by hadith. And they are implemented. To understand more on how in islam there is death penalty for certain things, you neeed to ask someone more knowledgeable than me because my knowledge is very limited.