The clause...
The clause, "and Allah sees what you do", explains the reason for the prohibition contained in the words, "be not like those who disbelieve". In the clause, "they would not have died and they would not have been slain", death is mentioned before slaying; the sequence follows that of the preceding clause, "when they travel . . . or engage in fighting".
Also, unlike slaying (which is an abnormal happening) death is a natural and normal phenomenon; therefore the normal was mentioned before the abnormal. The verse, in short, admonishes the believers not to be like the unbelievers; they should not say about someone who dies outside his home town or among strangers, or is slain when engaged in jihad, that if he had been with them he would not have died or been killed.
This type of talk throws one into mental agony and divine punishment - it is the intense grief put in their hearts. Moreover, it emanates from sheer ignorance: Being near them or away from them neither gives life nor causes death. Giving life and death is among the affairs exclusively reserved for Allah - the One Who has no partner or colleague. Therefore, the believers should fear Allah and not be like the unbelievers; and Allah sees what they do.
QUR'AN: And if you are slain in the way of Allah or you die, certainly forgiveness from Allah and mercy is better than what they amass: Obviously, "what they amass", refers to property and wealth as well as to its concomitants, as these are the best objects of desire in this life. This verse mentions martyrdom before normal death, because being slain in the way of Allah is nearer to divine forgiveness compared to other deaths. This fine point has caused this change of sequence.
Thereafter, the next verse, "And if indeed you die or you are slain. ..", reverts to the normal pattern, putting death before slaughter, because there is no such fine point there to justify any change. QUR'AN: Thus it is due to mercy from Allah. . . should the believers rely: "al-Fazz" (rude, rough); hard-heartedness metaphorically means unkindness, ill-nature and intolerance; al-infidad (to disperse).
The verse turns away from the believers, addressing the Messenger of Allah (s.a.w.a.) directly, although the real import is as follows: It is due to Our mercy that Our Messenger is lenient to you; that is why We have ordered him to pardon you, ask forgiveness for you and take counsel with you in the affair, and to place his trust in Us when he has taken any decision.