I think your prophet made it quite clear:
Allah's Apostle said, "The blood of a Muslim who confesses that none has the right to be worshipped but Allah and that I am His Apostle, cannot be shed except in three cases: In Qisas for murder, a married person who commits illegal sexual intercourse and the one who reverts from Islam (apostate) and leaves the Muslims."
—
Sahih al-Bukhari,
9:83:17, see also
Sahih Muslim,
16:4152,
Sahih Muslim,
16:4154
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 said, 'Don't punish (anybody) with Allah's Punishment.' No doubt, I would have killed them, for the Prophet said, 'If somebody (a Muslim) discards his religion, kill him.'"
—
Sahih al-Bukhari,
4:52:260
A man embraced Islam and then reverted back to Judaism. Mu'adh bin Jabal came and saw the man with Abu Musa. Mu'adh asked, "What is wrong with this (man)?" Abu Musa replied, "He embraced Islam and then reverted back to Judaism." Mu'adh said, "I will not sit down unless you kill him (as it is) the verdict of Allah and His Apostle."
—
Sahih al-Bukhari,[/QUOTE
I think the Quranic verses are very clear. Many of the sayings attributed to the Prophet are fabricated or twisted or interpreted out of context. It was not narrated that the prophet killed a turn coat. You need to differentiate between two apostates. The first one reverted from Islam and joined Islam fighters. Since Islam was in war in its early stage launched by the infidels to eradicate it, killing these fighters on capturing them was the rule. The other one, reverted but did not launch anti-islam activity, no one would touch him. What said by Quran comes first, next to this the authenticated prophet action interpreted in its context.