Various reports make clear that the assailant, apparently a native Turkish speaker, was believed to be providing security for the Ambassador. There are conflicting reports about whether the shooter was a security guard provided by the Turkish government, a private security guard tasked with protecting the Ambassador or simply someone who passed himself off as one or the other to carrying out the assassination. Whoever he is or was news reports also say the assailant has been "neutralized" though there is no confirmation of precisely what that means.
The assailant spoke first in Arabic and later in Turkish. But Turkish speaking journalists confirm that the assailant spoke Arabic with a Turkish accent and Turkish as a native speaker. Likely he was an ethnic Turk but almost certainly a Turkish national.
The final data point is that the assailant explicitly referenced the siege of Aleppo as the motive for his attack. Russia of course has been heavily involved in the siege and indeed has made possible the Assad government's retaking of the city. Whatever the identity of the assailant, having a foreign ambassador shot down undefended like this in the national capital will put the Erdogan government in an extremely difficult position vis a vis Russia.
Late Update: 1:16 PM: For roughly 45 minutes there have been reports by journalists on Twitter that the gunman was Mert Altintas, a Turkish police officer. Reuters has now confirmed that the assailant was an off-duty police officer.