The hadith content is authentic it has been compiled in many hadith compilations and at least three different sahabah with different narrator chains. So the meaning of the hadith fit well!
By Abdullah ibn Amr() in sahih al-Bukhari 1 and 2 and in sunan ibn Majah.
By Abu Bakrah () in sunan an-Nasai and sunan abi Dawod.
And by abu Hurrairah () in sunan ibn Majah.
All these ahadith say that any non-Muslim under the protection of Muslims is haram for killing this applies also for those who enter a Muslim country for business etc. as they have basically been allowed to enter, Muslims should protect them!
Now to at-Tabarni's narrations:
I found this one in his as-Sagheer and this one in al-Awsat which both go ahead with the content of this hadith from sunan ibn Majah which is qualified as sahih and the hadith is also in Musnad Ahmad and others such as al-Hakim in his mustadrak and an other version in at-Tabarni's as-Sagheer, which has a better narrator chain, but the issue is that this is a unique chain. Note that as-Suddi السدي (a common narrator of both versions of at-Tabarni is considerd as da'if by some scholars like ibn Ma'yn ) while the common narrator of al-Musnad and sunan ibn Majah Abdulmalik ibn 'Omayr عَبْدِ الْمَلِكِ بْنِ عُمَيْرٍ is "only known" for mixing narrations.
So the versions which include as-Suddi of at-Tabarani are less reliable than the others. And the one with good and trustworthy narrators has no backup by any other narrator chain in any other hadith compilation. Also note that the narrations compiled by other authors than at-Tabarani exclude the part "Even if the one who was killed was a disbeliever"! Al-Bazzar said that this addition only appears in as-Suddi's narrations!