The Frankish conquest of Saxony was noticeably difficult. It's not that Saxons benefited from enough resources or men to oppose Charlemagne on an equal standing (although the 4,500 men taken in 782 might have been only a small part of their overall capacities) but their capacity to revolt when the king had to deal with other matters and remove his forces dealing with other campaigns : several Merovingian kings, as well as his father, imposed a tribute on Saxons , but never really managed to firmly attach the region to their realm, something Charlemagne was eventually resolved to do, as Saxons regularly raided over eastern Francia.
His goal wasn't conquest for conquest's sake, but to reinforce a region that had escaped Frankish dominance for a long time and only put pack into obedience by his grand-father and his father by regular campaigning, such as what probably happened to Thuringia in the early VIIIth century, and violent events such as the massacre of Alemanic nobility by his uncle Carloman in 746 at Cannstatt, Bavaria keeping an uneasy autonomy thanks to its remoteness and alliance with Lombards (and, tentatively, with Aquitains) but forced to regularly bend the knee in the VIIIth century.
Saxony wasn't the only region that was never really part of the Merovingian realm (although understandable as part of his "outer" sphere at times) but were regularily involved in conflicts with Franks. Frisians, led by a probably confederal king, were a pagan people living in modern western Netherlands, that regularly clashed with them for the control of Meuse and Rhine trade, raiding Frankish borders or serving as mercenaries during civil wars. They were only defeated by Charles Martel after a series of military defeats, more or less superficial submissions, revolts and eventual annexation.
Warfare wasn't the only mean available to Carolingians, and their alliance with the Frankish, and eventually Roman, church allowed them to use conversion both as a display and maintain of vassalage in Germania; new fortifications, garrisons, bishoprics and monasteries allowed to lock down, but also provide important military and economical centers to peripheral regions lacking such: the monastery of Fulda seems to have played such a role in the 770's (incidentally built over an earlier Frankish fortification) and his abbot, St Sturm, having noticeable military and administrative skills.
It's is not really clear what Charlemagne envisioned for Saxony in long-term but his aggressive strategy was certainly made in perspective of a Saxon subjugation : rather than only addressing the border insecurity in a fairly stable and more economically important Frankish Germania, it's possible that integrating Saxony in the empire (although probably without that much of a clear idea of what it would like) was the eventual end goal. More than just using tributes and loot to redistribute wealth and honors to a clientelized aristocracy, converting Saxons and subjugating them (were essentially participating now to the same movement) was probably the key allowing the development of eastern lands into a firm Frankish control, including parts that were never even somewhat within Frankish sphere.
Very long story short, Charlemagne's forces were regularly able to advance against Saxons and to settle their presence trough military and religious establishments; making great military displays (such as the destruction of Irminsul, or their advance on Lippe River in 776) whose goal was to force local elite to surrender when seeing they were facing impossible odds. This worked fine enough, with conversions and the creation of sort of "pro-Frankish" faction among them, but utterly failed to prevent Saxons to counter-attack,rebel, burn strongholds and monasteries,and raiding Frankish Germania, which is what happened in 773, 776 and 778 (quite possibly with the support of Charlemagne's foes, especially Bavarians, Lombards and Byzantine whose advisors might have teached Saxons how to build siege engines).
While the Diet of Paderborn on 777 would seemingly have consecrated the Christianisation and Carolingianisation of Saxony, by baptizing Saxon nobles, integrating within a religious, legal and political Frankish framework and forced them to swear fidelity to Charlemagne,the dynasty and the realm by making clear that any violation would be harshly punished; the revolt of 778 appeared less as organizing an army as previously, than a whole high-scale guerilla in what was a land largely made up of marshes and forests.
Charlemagne eventually encountered the same problems that Germanicus, Tacitus and Varus did in their time up to Franks having their own Teutoburg at Suntel in 782 : there, forty counts, twenty officers, Gello the constable and Adalgis were killed. What was most probably a defeat (or at least a damaging stalemate) had an important echo in Francia, prompting Charlemagne to come in Saxony, to gather the local nobles and requiring them to give away anyone who was in relation or helping the rebels, and received at least 4,500 of them (possibly who participated to the Battle of Suntel) which were all killed on the spot.
This was less a fit of murderous rage, but a convenient way to deal with what looked like an insoluble task : pacifying the region by either integrating local nobility into a Frankish and Christian realm; either dealing with pagan Saxon rebels.By requesting the first to give him culprits, Charlemagne symbolically and effectively created a rupture of solidarity and support among Saxons while displaying the bearings of an Old Testament king (Frankish kingship being associated with Biblical kingship since the VIth century), likening the treatment of Saxons with the treatment of people as Amalekites.
It was followed by the really harsh Capitulaire De partibus Saxoniae which threatened any religious and political crime with death sentence, and a period of intense pressure on Saxons,which didn't really resolve much immediatly : the end of the wars in the 790's owes a lot to an exhaustion of rebel Saxons (Widukind ended converting and submitting) as well as Frankish undergoing a softer policy of reconciliation to at least pacify the region in the 800's.
It was a long introduction for a fairly short answer, but giving we're talking of warriors and local nobles, it's unlikely the executed persons were women or children unable to bear arms. Most of them were rather victims, beyond the consequences of war, religious persecution and guerilla, of the important deportations of Saxons that took place in the following decades, sending whole families elsewhere in Francia.