If you have a belt of about 50-100 rounds of 50 caliber in the box, and no more handy, and nowhere to retreat, and you look out at a crowd of 10,000 coming at you waving swords, etc....
You do the math, and realize that after your belt is expended, you will be torn to pieces, literally, ripped to shreds.
And no 72 virgins for killing other Muslims.
That would explain choosing not to fire, but that doesn’t fit what they say happened.