Is it true that the Prophet, peace and blessings be upon him, swore to mutilate seventy of his enemies when he saw the mutilated body of his uncle Hamza, and if so, how would the Prophet repay evil with evil in revenge?

Scholars cite a Hadith (definition) in interpreting the verse: "And if you punish [an enemy], then punish with an equivalent of that with which you were punished. But if you are patient—it is better for those who are patient." (An-Nahl/126). This Hadith is weak and unproven, as it is narrated from three Companions through unsound chains.

The first Hadith, from Abu Hurairah: It is weak due to Salih bin Bashir Al-Murri. The second Hadith, from Ibn Abbas: It is narrated through two weak chains. The first contains three weak narrators, and the second contains ambiguity and اضطراب (instability/disruption). The third Hadith, from Qais bin Sa'd: It is very weak due to Muhammad bin Yunus Al-Kadimi, who is accused of fabricating Hadith.

In conclusion, all three Ahadith are weak. What is established is that some of the Companions vowed to mutilate the bodies of the polytheists in revenge for their slain, but Allah forbade them from doing so and called them to patience. It is authentically reported from Ubayy ibn Ka'b that the Ansar said this after [the Battle of] Uhud, and the verse was revealed thereafter.