Is the verse: "O you who have believed, do not be like those who disbelieved and said about their brothers when they traveled through the land or went out to fight, 'If they had been with us, they would not have died or been killed' - so Allah makes that a regret within their hearts," an evidence that whoever dies by being killed would have inevitably died even if they had not been killed?

The slain person dies at their appointed time, according to the general consensus of Muslims. Sheikh Al-Islam Ibn Taymiyyah clarified that divine decree preceded the death of the slain person by the very cause of their death, and not by another. He explained that asserting they would have died by another cause, or that they would not have died if that cause were absent, is ignorance, because divine decree predetermined their death by that specific cause. As for the verse: "{If they had been with us, they would not have died or been killed}," it does not provide conclusive evidence for either party. Al-Sa'di explained that Allah forbids believers from resembling the disbelievers who oppose divine decree and say: "{If they had been with us, they would not have died or been killed}," and that Allah refutes them by saying: "{And Allah gives life and causes death}." He alone has this power, and no caution can avert destiny.