We simply don't have sufficient evidence to be certain about any of this. This is because the evidence that survives, in the form of the remains of "bog bodies", is highly varied in terms of both geographical location and actual manner of death; in addition, the bodies we have were discovered entirely by chance and likely form only a tiny fragment of a much larger whole, and we're unaware of how big or how similarly treated that "whole" might be. There is essentially no written evidence as to either acts or motivation, bar a couple of interesting, but third-hand or more, comments from Roman sources, and the actual nature and severity of the wounds inflicted on the recovered bodies that we do have is complicated by the problem that many were recovered only after being exposed by the modern peat-cutting industry, which uses enormous cutting blades to chop up peat – hence it is often uncertain whether the "wounds" that have been found on some of these ancient bodies were actually inflicted in the course of their disinterral. Finally, there is essentially no firm evidence to show whether bodies that do, on balance, appear to have been the subjects of violence were those of "human sacrifices", criminals, murder victims, casualties of battle, or people who had got into fights.

Very plausibly, the set of bodies we do have collectively represent people who had experienced most, if not all, of the above. About the only thing we can say is that the bog bodies that do seem to reveal some evidence of suffering violence that might suggest some form of ritual had occurred are not nearly so uniform as the question suggests – there are definitely differences in manner of death, age, sex, and social status that raise more questions than they answer.

I covered the questions you are raising in an essay written a few years back that I will paste below – it begins with a discussion of Old Croghan Man. The original is available here and is fully illustrated.

The bodies in the bogs

In an ancient bog at the foot of a fairy-haunted hill, peat-cutting work lays bare the body of a giant. Carbon dating suggests that the man died at the height of the Iron Age, around 275 B.C.; forensic examination shows that he died hard, stabbed through a lung and then decapitated with an axe. After killing him, his executioners chopped his body in half at the diaphragm, and at some point, perhaps while he was still alive, they also inflicted two pairs of unusual wounds on him. Deep cuts almost severed both his nipples, and his arms were vigorously pierced so that twisted lengths of hazel withy could be threaded through from side to side, presumably to pinion him. After that, his mutilated torso was sunk in a pool where, over the years, bog moss grew up to cradle and cover him, until he became part of the mire itself.

As the dead man’s assailants were most likely perfectly aware, the unusual properties of the bog and the moss combined to preserve his remains. The sour waters of high bogs are as acidic as vinegar, and they support practically no life, yet they contain bog oak – which deeply tans organic matter – and sphagnum moss, which uniquely binds both nitrogen and oxygen, inhibiting bacteria. Trapped in this nutrient poor, anaerobic environment, human remains are preserved almost intact; bones may be leeched and gradually demineralise, but flesh and wood, horn, fur, hair and textiles can and do survive for millennia. So when ditching work uncovered the torn remains that archaeologists now call “Old Croghan Man” outside the little village of Croghan, in County Offaly in the heart of Ireland, investigators could still make out the pores on his skin and inspect the well-manicured fingernails that showed that he had done no manual work and hinted at high status. They could calculate that he had once stood 6 feet 5 inches [1.95m] tall: a great height now, freakish for his day. And they could feel reasonably certain that that height had been made possible by an unexpectedly rich diet, predominantly comprised of meat.

Beyond that, though, almost everything is mystery. We can only speculate as to why the giant’s life was cut so short, and why he died while in his twenties, at the height of his physical powers. We cannot know why the people who killed him felt it necessary to inflict such violence on his body. Nor do we properly understand what the peculiar mutilations that they added to his torso meant: what magic they were intended to perform, or what catastrophe they were intended to commemorate – or, perhaps, prevent.

What we can say is that Old Croghan Man must have been special in some way. His size and strength would certainly have made him physically quite different – he must have been, Valerie Hall suggests, “the golden boy of his tribe. Those big, capable hands… even in death, he oozes confidence, status, presence.” He did not die a normal death, nor was his body handled in a manner typical of his time and place. Early Iron Age burials usually involved cremation, while late ones substituted interral, almost always with grave goods of some sort. Bog burials seem to have been rare, though of course we cannot be sure how many there were. Estimates run into thousands, yet, while archaeologists are careful to point out that there are several different sorts of bog body – and that some of the people whose remains survive apparently died natural deaths – a residue of several dozen hacked and mutilated corpses suggest that other, highly specific, motives were occasionally at work. The members of this last group combine thought-provoking characteristics (babies are under-represented; the young and people with obvious deformities or disabilities are heavily over-represented) with the preserved marks of such extreme violence that it amounts sometimes to overkill. It has been suggested that the evidence shows that their deaths came in the form of stage-managed ceremonies: a theatre of death that culminated in human sacrifice.