I was wondering; is it accurate to say regarding a killed or murdered non-Muslim possible fate that if someone is killed unfairly, then their life has been cut short, which interferes with the natural flow of destiny, and henceforth that person had no chance to come to belief later in life because their life was cut short, and then due to the possibility which did not come to light, that person may be given Heaven as a reward because Allah's mercy is twice that of His wrath. Have scholars said anything about this?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

The statement which you mentioned is absolutely wrong without any doubt. If the true religion reaches a person in a way that the likes of him understand, then the evidence has been established against him. So if he dies or is killed as a non-Muslim, he will abide in Hellfire eternally – we seek refuge in Allah from that. 

Allah says (what means): {[We sent] messengers as bringers of good tidings and warners so that mankind will have no argument against Allah after the messengers. And ever is Allah Exalted in Might and Wise.} [Quran 4:165]

There is no single person among the people of Hellfire except that he will ask to return to this worldly life – whether his life was long or short – in order to believe in the truth and to act according to it, but his request will be declined. He will be blamed about his lifespan that he spent in this worldly life and will be told that it was sufficient for him to accept the truth and to believe in it but he did not do so. Allah says (what means): {And for those who disbelieve will be the fire of Hell. [Death] is not decreed for them so they may die, nor will its torment be lightened for them. Thus do we recompense every ungrateful one. And they will cry out therein, "Our Lord, remove us; we will do righteousness – other than what we were doing!" But did We not grant you life enough for whoever would remember therein to remember, and the warner had come to you? So taste [the punishment], for there is not for the wrongdoers any helper.} [Quran 35:36-37]

Ibn Katheer said in his interpretation of the above verses:

“{Thus do we recompense every ungrateful one} meaning: this is the recompense of everyone who disbelieved in his Lord and denied the truth. {And they will cry out therein} meaning: they will call out in the Fire, beseeching Allah with their voices: {Our Lord, remove us; we will do righteousness – other than what we were doing!} meaning: they will ask to go back to the worldly life so that they can do something different than the deeds they did. But Allah, may He be glorified, knows that if He sent them back to this world, they would go back to what they had been forbidden to do, and He knows that they are lying, so He will not respond to their plea. This is like the verse where Allah tells us that they will say: {Then is there any way to get out (of the Fire). (It will be said:) This is because, when Allah Alone was invoked, you disbelieved (denied), but when partners were joined to Him, you believed!} [Quran 40:11-12]; i.e. there will be no response to you because you were like that; if you were to return to this world, you would just go back to that which you had been forbidden to do. Allah says here: {But did We not grant you life enough for whoever would remember therein to remember} meaning: did you not live long enough in the world that if you were to be among those who would benefit from the truth, you would have benefited from it during your lifetimes.” 

If what you mentioned in the question is true, it would have been a justification for whoever lived a short life, even if he died of natural causes (not killed), and there is no doubt that this is false. The most that could be said in this regard is that a long life in disbelief is an increase in accountability and punishment, so if someone’s life is long and he disbelieved, then he is worse than the one who disbelieved but his life was short.

Abu Bakrah Ath-Thaqafi narrated that a man asked the Prophet “O Messenger of Allah, who is the best of people?” He replied, “The best of people is the one who led a long life and performed good deeds.” The man asked, “And who is the worst of people?” He said, “The worst of people is the one who led a long life and performed bad deeds.” [Ahmad, At-Tirmithi and Ibn Maajah - At-Tirmithi said: hasan saheeh (good or sound); Al-Albaani and Al-Arnaa’oot graded it hasan (good)]

On the other hand, killing neither shortens one’s life nor contradicts destiny; rather, it is part of destiny. It is a manifest error to say, “If someone is killed unfairly, then their life has been cut short, which interferes with the natural flow of destiny.” The ideology of Ahl As-Sunnah is that the person who is killed has died with accordance to the destiny that Allah decreed (for him).

As-Saffaareeni said in Lawaami’ Al-Anwaar Al-Bahiyyah, “In brief, the person who is killed died according to his appointed term which Allah has set for him, and his death is neither delayed nor advanced one single moment, as Allah has decreed the appointed time of the slaves with Knowledge without hesitation: {So when their time has come, they will not remain behind an hour, nor will they precede [it].} [Quran 7:34]…”

Haafith Hakami said in Ma'aarij Al-Qabool,

“Belief in death involves some matters ... including that each and every one has a limited time which he neither exceeds nor falls short of. Allah knows all this with His Knowledge, which is one of His Attributes, and which the pen was commanded to write on the day Allah created it, then the angel wrote it about each person while in the womb of his mother by the commandment of His Lord Almighty when He created the very Nutfah (sperm drop); at which place his death will be and at which time it will be. It can never be increased, nor decreased, nor changed, nor altered from the Knowledge of Allah that preceded it, with His Predestination and Decree; and that every person who has died, or who is killed, or burned, or drowned, or died with any given reason, he does not delay from it nor does he precede it for the blink of an eye; and that the reason that led to his death is the one which Allah predestined and decreed for him; and that he has no other alternative nor escape from it.

Allah says (what means): {And it is not [possible] for one to die except by permission of Allah at a decree determined. And whoever desires the reward of this world – We will give him thereof; and whoever desires the reward of the Hereafter – We will give him thereof.} [Quran 3:145]

Allah also says (what means): {Say, "Even if you had been inside your houses, those decreed to be killed would have come out to their death beds."} [Quran 3:154] ... There are many other verses in this regard…”

Allah knows best.