We read in Saheeh Bukhari, Vol. 4, Book 52, Hadith 286:
Narrated Salama bin Al-Akwa`:
"An infidel spy came to the Prophet while he was on a journey. The spy sat with the companions of the Prophet and started talking and then went away. The Prophet said (to his companions), 'Chase and kill him.' So, I killed him." The Prophet then gave him the belongings of the killed spy (in addition to his share of the war booty). 
Notice how the hadeeth portrays the Prophet as killing someone who wasn't even a threat, for the man was walking away. How can we accept such a portrayal of the Prophet?
This objection is rather silly given that the hadeeth clearly states that the person in question was a spy. This spy came to gather information for the kuffar who were at war with the Muslims.
How does the person objecting know that the information the spy gathered whilst listening to the discussions of the Muslims wouldn't have posed a threat to the Muslims eventually? If the spy were a threat, then the Muslims would have been weakened if he escaped .
Furthermore, killing this spy would have sent a message of deterrence to the enemies of the Muslims so that they would think twice before spying next time.
In summary, this objection is rather shallow and baseless.
1) Also see Sunan Abu Dawud, Book 14, Hadith 2647
2) Abdullah bin Saalih 'aal Bassam in his commentary of 'Umdat al-Ahkaam said regarding one of the benefits derived from this hadeeth: قتل العن الذي يبعثه الأعداء ليخبر المسلمين، ويتعرف على أحوالهم، لأن في تركه ضرراً على المسلمين بالإخبار عن حالهم، ومكان الضعف منهم، والدلالة على ثغراتهم
بخلاف الرسل، فإنهم لا يُؤذوْن، لأنهم دعاة سلام وصلة التئام، وهذا من محاسن الإسلام.