How can one respond to a non-Muslim colleague who cites historical events and hadiths—which he considers authentic—to question the mercy of Prophet Muhammad, peace and blessings be upon him, and attributes harsh characteristics to God based on Quranic texts?

Allah Almighty sent His Messenger Muhammad, peace and blessings be upon him, as a mercy to the worlds. Whoever accepts it attains mercy, and whoever rejects it is deprived of it. Allah Almighty has clarified that the Prophet is compassionate and merciful to the believers. As for those who reject mercy and spread corruption in the land, like the incident of the "'Ukleeyyin" who murdered and stole after embracing Islam, cutting off their hands and feet was a rooting out of their corruption and a protection for society. This is known as the hadd al-hirabah (punishment for brigandage). Similarly, the killing of the Jews of Banu Qurayzah was due to their breach of covenant and their alliance with the confederates to eliminate the Muslims at a critical time, and Sa'd ibn Mu'adh's ruling on them was their own choice. As for the claim of raping women, it is false, and the Prophet forbade killing women and children in wars. The fact that some of them embraced Islam after captivity is evidence of Allah's mercy towards them. The people of disbelief and corruption will not be included in Allah's mercy on the Day of Resurrection, because His mercy is specifically for the righteous believers. Islam has established the proof for everyone, and whoever chooses disbelief should blame no one but himself.