What can the victim say that will make the Muslim stop and think?
I testify that there is no God but one God called "The Lord God" or in other words "Allah" (The God), and I also bear witness that a man called Muhammad was a prophet of God.
If that muslim is not baffled i.e. if he did not understand what you just said, say the same above line in Arabic,
ašhadu ʾan] lā ilāha illā l-Lāh wa [ʾašhadu ʾanna] Muḥammadan rasūlu l-Lāh (Source Shahada)
You just asked, what will make him stop and think, then this is what at least makes him stop and think for a while. Later, if he will kill or will not kill will entirely depend on the circumstances.
I believe that a Muslim that understands and follows their belief is just as loving and gentle as the rest of us.
When you yourselves, say so, why not give it a try? Explore, ponder, and think of believing.
EDIT: You can also remind him with this Ayah from Qur'an,
Say, [O Muhammad], to those who have believed that they [should] forgive those who expect not the days of Allah so that He may recompense a people for what they used to earn. - Qur'an 45:14
This has a higher probability that the killer might completely stop, but doesn't guarantee that because as said already, it depends on various circumstances like if, the person is deemed to receive death penalty by the Islamic law, then no one can help.