Praise be to Allaah.  

Al-Bukhaari (4073) and Muslim (1793) narrated that Abu Hurayrah (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The anger of Allaah is most intense against a man who is killed by the Messenger of Allaah for the sake of Allaah (on the battlefield).” 

Al-Nawawi said: 

“For the sake of Allaah” excludes one whom he kills as a hadd punishment or by way of legal retaliation (qisaas), because whoever is killed by the Prophet (peace and blessings of Allaah be upon him) on the battle was intending to kill the Prophet (peace and blessings of Allaah be upon him). 

It is not known that the Prophet (peace and blessings of Allaah be upon him) killed anyone among the mushrikeen with his own hand apart from Ubayy ibn Khalaf. 

That was narrated by Ibn Jareer and al-Haakim from Sa’eed ibn al-Musayyab and al-Zuhri (may Allaah have mercy on them). Ibn Katheer said in his Tafseer (2/296): its isnaad is saheeh. 

Ibn al-Qayyim said – when discussing the battle of Uhud –  

The Prophet (peace and blessings of Allaah be upon him) turned towards the Muslims, and the first one who recognized him beneath his armour was Ka’b ibn Maalik, who shouted at the top of his voice, “O Muslims! Rejoice! This is the Messenger of Allaah (peace and blessings of Allaah be upon him)!” He gestured to him to be quiet, and the Muslims gathered around him and went with him to the mountain pass where he had camped. Among them were Abu Bakr, ‘Umar, ‘Ali, al-Haarith ibn al-Simmah al-Ansaari and others. When they got close to the mountain, Ubayy ibn Khalaf, who was on a horse of his whose name was al-‘Iwadh, caught up with the Messenger of Allaah (peace and blessings of Allaah be upon him).  The enemy of Allaah had claimed that he would kill the Messenger of Allaah (peace and blessings of Allaah be upon him) whilst riding this horse. When he got close to him, the Messenger of Allaah (peace and blessings of Allaah be upon him) took a spear from al-Haarith ibn al-Simmah and killed him with it. It hit him in his collarbone and the enemy of Allaah retreated in defeat. The mushrikeen said to him, By Allaah, there is nothing wrong with you. He said, By Allaah, if what is wrong with me had happened to the family of Dhu’l-Majaaz, they would all have died. He used to feed his horse in Makkah and say, I will kill Muhammad riding it. News of that reached the Messenger of Allaah (peace and blessings of Allaah be upon him) and he said: “Rather I will kill him in sha Allaah.” When he stabbed him, the enemy of Allaah remembered that that the Prophet (peace and blessings of Allaah be upon him) had said, “Rather I will kill him,” and he was certain that he would die from this wound. And he died from it in Sarif on his way back to Makkah. 

Zaad al-Ma’aad, 3/199 

And Allaah knows best.