If those who were shot were innocent, it follows ineluctably that those who shot them either acted culpably and unlawfully or made multiple errors of judgment of truly appalling proportions.
That is the nub of the issue for the new British judicial inquiry into Bloody Sunday. It is also the yardstick by which the nationalist community of the North, and of Derry in particular, will assess the integrity of the inquiry's findings.
Yet the issue may be avoided. There is no jury involved, at this stage, to pronounce guilt or innocence. The verdict, in those bald terms, may be left implicit in whatever description of the fateful events is assembled by Lord Saville and his judicial colleagues.
If the evidence is rigorously gathered and fairly assessed, the synthesis should point compellingly to certain conclusions. But it may well be left to the discretion of the British DPP to decide whether or not such conclusions should be tested, in individual cases, in a court of criminal law.
The inquiry may not, indeed judicially must not, be influenced by the fact that thousands believe firmly that murder was done. Nor is it likely, however desirable it might be, to deconstruct critically the work and conclusions of the Widgery tribunal.
It will be enough for most people if the inquiry addresses resolutely and impartially what it has to do, rather than what any of the political, community or institutional interests looking on may desire it to do.
By its very nature, such an exercise should discredit Lord Widgery's sophistry even more unequivocally than the very decision to institute the new inquiry has done.
Yet by no means should Lord Widgery's ritual three-week process be set aside completely. For one thing it provided the only body of sworn evidence, heard in public and tested by cross-examination, still extant in relation to Bloody Sunday.
That can and should still be drawn upon, irrespective of his flawed and selective final report. That evidence was extremely limited in regard to civilian eye-witnesses, mainly because most people did not trust his bona fides and refrained from offering themselves.
But, such as it was, it was fresh and much of it was cogent, even damning.
Accounts such as that given before Widgery by the respected Guardian journalist, Simon Winchester, must remain of considerable value to the new inquiry. Winchester, who had written that he was personally fired upon by a group of soldiers, was subjected to scathingly hostile cross-examination aimed - with some superficial success - at forcing him to admit that there might just be another explanation for what he had seen and experienced.
Widgery heard evidence from just 30 Derry civilians, out of the many hundreds who had witnessed potentially-relevant incidents on the day. Seven priests gave evidence, and 21 press and television reporters and photographers. That evidence, in spite of Widgery's subsequent wordcrunching, is still highly relevant today.
Widgery also heard no fewer than 48 soldiers and RUC officers.
The pat accounts of the Parachute Regiment soldiers who, to a man, swore that they had shot only people whom they had clearly identified as gunmen or bombers, prompted even Lord Widgery to split hairs in his conclusions, viz "some showed more restraint in opening fire than others" (whatever that might be taken to mean).
If the new inquiry is perceived as acting in good faith, a great body of additional civilian eyewitnesses may offer themselves. But Lord Saville and his colleagues will have to trawl energetically, using duress and subpoena, if they are to extend - as they must surely do - the range of military and police witnesses called in.
It is not clear, at time of writing, whether a verbatim record was made of the oral evidence given to the Widgery tribunal. No such transcript seems to have surfaced publicly in 26 years.
Failing its availability, the most detailed reports of the evidence on public record would be those carried in The Irish Times from day to day of the tribunal, frequently running to two pages in length.
Another vital piece of evidence barely touched on by Widgery is the film shot from the British army helicopter which hovered above the Bogside during the firing.
A selective section of this film - how selective, it is impossible to say - was screened once in a darkened room for viewing by legal counsel and a few press representatives.
It showed the rapid deployment of the Paras into Rossville Street, the crowd fleeing, and the faint muzzle discharges of the rifles. No attempt was made by Widgery to have this film subjected to close scrutiny; no still shots or enlargements were made available.
Don Mullan's book, Eyewitness Bloody Sunday, and the other witness accounts compiled and analysed in the Government's assessment published yesterday, clearly indicate how vital it is that the fresh British inquiry should establish which army units fired from Derry's walls, how many shots they fired, who fired them, at whom they were fired, and why.
Widgery completely failed to adduce that this had occurred at all.
The closest he came, after accepting without question that an ammunition check had indicated that only 108 rounds were fired by 1 Para, was the enigmatic statement: "About 20 more rounds were fired by the army in Londonderry that afternoon, but not by 1 Para and not in the area with which the tribunal was primarily concerned."
This is plainly a crucial point upon which the new inquiry must seek elaboration. It might also probe whether shots were fired from a British army observation post high up on top of the Embassy Ballroom, a few hundred yards away in the city centre.
The entry and exit holes of a bullet which penetrated a metal box stanchion on the side of Rossville Flats indicated such a trajectory.
The overwhelming thrust of the photographic and oral evidence to Widgery was that all of those killed or wounded by soldiers' bullets were in the open and fully visible, either running away, standing or crouched down.
If some shots were fired at the advancing Paras (other than by the youth carrying a small handgun in the courtyard of the flats, as described by Father Edward Daly), they were certainly fired from pre-chosen cover points rather than by the fleeing victims.
The inquiry will have great difficulty establishing this, as presumably no IRA or "independent" republican snipers will make themselves amenable to the inquiry. Such an area of doubt may be used - as Widgery blatantly did - to contrive a degree of justification for the soldiers' firing. But when the position and fate of each victim are examined precisely, as the extended evidence now permits, the judicial assessment must establish whether or not their deliberate targeting can be justified, irrespective of any extraneous firing at the Paras.
The credibility of the new inquiry may indeed turn upon whether or not it unequivocally establishes (as it now should have available adequate and exact enough evidence to do) the total innocence of all those killed and wounded.
Lord Widgery carefully fudged his findings on this matter, plainly perceiving that the establishment of innocence on one side would logically imply guilt on the other.
Astonishingly, he did not bother even to obtain evidence from all of the injured as to the circumstances in which they were shot. Only seven of the 13 known wounded appeared before his tribunal.
He remarked in his report: "I did not think it necessary to take evidence from those of the wounded who were still in hospital."
All of these matters may be elucidated by the new inquiry. It must also address the inconsistencies between the statements the Paras gave to military police on the evening of Bloody Sunday and their statements at the tribunal.
At first glance it would seem that the pivotal task of the inquiry will be to test to the limit the accounts of the soldiers against the great mass of forensic and other evidence which Lord Widgery chose not to explore.
But a more delicate, and not necessarily totally overt, area of their duties will be to probe the higher levels of the chain of military and political command.
Was it pure chance, and the psychopathic tendencies of some Paras, that led to the slaughter? Or was there an overriding, opportunist and recklessly imprecise political strategy that created the circumstances in which military discipline, at a certain point, could spin out of control?
Small wonder that one Derry woman, interviewed on RTE television on Thursday evening, commented: "It remains to be seen."