My understanding of Valhalla is that only those that are killed on the field of battle are eligible to go to Valhalla and, in fact, only half of those go there. The other half go to Fólkvangr and are watched over by the Goddess Freyja as opposed to Odin. In terms of the human sacrifice undertaken by the Norse I'm making the assumption you're referring to Longboat burial ceremonies. Numbers of sacrifices vary according to sources and the grandeur of the ceremony but Ibn Fadlan, a Muslim visitor in the 10th century, reports that one thrall (slave) girl would volunteer to accompany her master to the afterlife. In a process involving ritualistic rape, intoxication and eventual stabbing she would join him upon his longship. However, the Sigurðarkviða hin skamma contains several stanzas in which the Valkyrie Brynhildr lays out the rituals for Longship burial in which it is stated that "Bond-women five shall follow him, And eight of my thralls, well-born are they, Children with me, and mine they were As gifts that Buthli his daughter gave" From the sounds of it only slaves were sacrificed, some were volunteers, some were forced and, in all likelihood, the sacrifices, being thralls, would not enter Valhalla even if they were to die in battle.

The subject of norse human sacrifice is interesting!

Pre-viking age many archeologist see the peat bodies, or bog bodies found in Denmark, as evidence of some type of ritualistic sacrifice or killings. They're from pre-history, most of them date back to the iron age. Obviously there is a lack of written accounts to interpret the cause of the bodies, and why they were strangled and put in a bog. Capital punishment is another, not unlikely, explanation.

/u/Frosticle has already mentioned Ibn Fallad, which is very interesting. He describes a Rus ship burial. The chieftain was already dead, but a thrall girl was killed as a retainer sacrifice. Ie, he would be able to maintain the same lifestyle in death as he had in life. They also sacrificed two horses, a hen and a cock, and put various weapons and grave offerings next to him. The slavegirl went willingly (according to ibn Fadlan).

Adam of Bremen described the other type of sacrifice, ie one performed for the benefit of the gods at a religious festival. Sorry about the wall of text, but I figure you might want to read it straight from the source, so to speak.

In Uppsala, at nine-year intervals, a general feast of all the provinces of Sweden. From attendance at this festival no one is exempted.3 Kings and people all and singly send their gifts to Uppsala and, what is more distressing than any kind of punishment, those who have already adopted Christianity redeem themselves through these ceremonies. The sacrifice is of this nature: of every living thing that is male, they offer nine heads,4 with the blood of which it is customary to placate gods of this sort. The bodies they hang in the sacred grove that adjoins the temple. Mow this grove is so sacred in the eyes of the heathen that each and every tree in it is believed divine because of the death or putrefaction of the victims. Even dogs and horses hang there with men. A Christian seventy-two years old told me that he had seen their bodies suspended promiscuously. Furthermore, the incantations customarily chanted in the ritual of a sacrifice of this kind are manifold and unseemly; therefore, it is better to keep silence about them

3 Scholium: When not long ago the most Christian king of the Swedes, Anunder, would not offer the demons the prescribed sacrifice of the people, he is said, on being deposed, to have departed "from the presence of the council, rejoicing" that he had been "accounted worthy to suffer reproach for the name of Jesus." 4 Scholium: Feasts and sacrifices of this kind are solemnized for nine days. On each day they offer a man along with other living beings in such a number that in the course of the nine days they will have made offerings of seventy-two creatures. This sacrifice takes place about the time of the vernal equinox.

Heimskringla tells of Swedish King Aun who sacrificed nine of his sons in an effort to prolong his life until his subjects stopped him from killing his last son Egil.

Tacitus writes of german tribes and human sacrifices, substituting the names of the gods with the roman equivalents:

Mercury is the deity whom they chiefly worship, and on certain days they deem it right to sacrifice to him even with human victims. Hercules and Mars they appease with more lawful offerings. Some of the Suevi also sacrifice to Isis. Of the occasion and origin of this foreign rite I have discovered nothing, but that the image, which is fashioned like a light galley, indicates an imported worship. The Germans, however, do not consider it consistent with the grandeur of celestial beings to confine the gods within walls, or to liken them to the form of any human countenance. They consecrate woods and groves, and they apply the names of deities to the abstraction which they see only in spiritual worship.

Saxo Grammaticus writes in Gesta Danorum:

I]n order to mollify the divinities he did indeed make a holy sacrifice of dark-coloured victims to the god Frø [Freyr]. He repeated this mode of propitiation at an annual festival and left it to be imitated by his descendants. The Swedes call it Frøblot.

Snorri Sturlason writes that king Domalde and king Olof Trätälja were said to have been sacrificed after years of famine.

Short Lay of Sigurd (Sigurðarkviða hin skamma), is an old norse poem, its a fragment of a now lost, longer poem. The ending is about how Brynhild's last wish is that Sigurd's pyre be built wide enough for both her and Sigurd. The pyre would be covered with shields, carpets and killed slaves. She requested that the slaves should burn fully decked beside Sigurd. Two were to be at his feet and two at his head. There were also to be a brace of dogs and a pair of hawks. Between Sigurd and Brynhildr they were to put the sword that lay between them when formerly they were sleeping together and they were called wedded mates.

Bond-women five
shall follow him,
And eight of my thralls,
well-born are they,
Children with me,
and mine they were
As gifts that Buthli
his daughter gave.

So, uhmm, your question! I kinda got lost there as I was writing.

You'd not go to valhalla by being sacrificed. For a retainer sacrife (like Ibn Ibn Fadlan), the idea seems to have been that you followed your master (or mistress) into the afterlife. How voluntarily it was is hard to answer, and it would be mostly speculation if I do, so I won't.

Typically, human sacrifices are an offering to a deity, a much better and more powerful offering than any animal. To gain the favour of the gods for the benefit of yourself and/or your community, and to keep the gods happy.

By the vikingage, the memory and tradition of human sacrifices seems to be there, but there are few accounts of it occuring as such anymore.

There are several possible afterlifes in Norse mythology.

Valhall and Folkvangr were reserved for those that died in battle. It is possible that the God Ull, or Ullr, son of Tor's wife Siv (and Tor's stepson) who was commonly associated with archery, huntin, skiing and dueling had his own afterlife at Ydalir (yew tree valley) for those that died through archery, were archers or hunters, or died in duels.

However, the main afterlife for those that did not die in battle was Hel, where the Godess of Death, Hel, reigned. When she was thrown down there the side she landed on was broken and she is since blue-black all over that side of her body.

She faces one hellish part of her realm with her blue-black side, where she has a serving platter called Hunger (Hunger), a knife named Svält (Famine), a bed called Sjukläger (Sick Bed) with a cover called Glänsande ofärd (Shining disaster) and a treshold called Fallande fördärv (Falling damnation).

However, on her fair side is the other part of her realm, sometimes called Helgafjäll (Holy Mountain) where the dead have warm hearths, plenty of food and ancestors and descendants meet again to speak of what they accomplished and get their questions answered.

So, to summarize;

Valhall, half of those that died in battle.

Folkvangr, the other half of those that died in battle.

Hel (the bad part), those that failed to live a moral life (rapists, murderers, thieves, robbers etc).

Hel (the good part/Helgafjäll), those that lived a good and moral life but did not die in battle.

And possible Ydalir for achers, hunters and duelists.

Nordic mythology was never centralised, and what we have is to a large extent how it was interpreted in Iceland at the time right before christianity took over, as written down by Snorre Sturlasson and a few other Sagas that have survived.

It is quite possible that the Geats or Svitjods believed that they would end up in Ydalir while the Norwegians and Icelanders believed in Valhalla.

To answer your question - if going by what Snorre Sturlasson wrote down, the Norse probably believed that those that were sacrificed, if they had lived a moral life up to that point, ended up on Hel's good side.