What do you call someone who dies for a good cause or an honourable reason?
An example would be a soldier who dies while saving his country.
A martyr. While the word hero could also be used, it can happen that the hero does an honorable thing and survives, while the word martyr always signifies that the person died.
You would say that such people had sacrificed themselves for the sake of others. That’s fine as a verb because it is used reflexively.
The problem with saying that they were “a sacrifice” as a noun is that it suggests they were sacrificed by someone else rather than giving up their lives for others.
Correcting this leads to the noun self-sacrifice, which the OED defines as:
Sacrifice of oneself; the giving up of one’s own interests, happiness, and desires, for the sake of duty or the welfare of others.
- 1805 Wordsw. Poems Sentim., Ode to Duty 54 ― Give unto me, made lowly wise, The spirit of self-sacrifice.
- 1843 Kingsley Lett. (1878) I. 101 ― What a strange mystery is that of mutual self-sacrifice! to exist for one moment for another.
- 1885 J. Martineau Types Eth. Th. I. i. i. §7. 226 ― Absolute self-sacrifice of the passions and imagination.
The OED also provides a bunch of derived terms that stem from self-sacrifice:
So self-ˈsacrificed pa. pple.; self-ˈsacrificer, a self-sacrificing person; self-sacriˈficial a. = next; self-ˈsacrificing ppl. a., making a sacrifice of one’s life, etc. (whence ‑ˈsacrificingly adv., ‑ˈsacrificingness).
- A. 1711 Ken Preparatives Poet. Wks. IV. 83 ― *Self-sacrific’d, his Father’s Will, And our Redemption to fulfil.
- 1900 Inscr. in Postmen’s Park, Aldersgate St., London, ― Mary Rogers, stewardess of the Stella, March 30, 1899, self-sacrificed by giving up her life-belt, and voluntarily going down in the sinking ship.
- 1668 H. More Div. Dial. iii. xxv. I. 467 ― Martyrs and *Self-sacrificers to but so faint a Shadow··of the first uncreated Perfection.
- 1903 Sat. Rev. 4 Apr. 421/2 ― It is usual for the self-sacrificer to be a consistently melodramatic person.
- 1855 Bailey Mystic, etc. 98 ― The painful pelican *Self-sacrificial.
- 1893 H. R. Reynolds in Life (1898) 473 ― Your self-sacrificial love to a great duty.
- 1817 Moore Lalla Rookh, Parad. & Peri (ed. 2) 149 ― That precious sigh Of pure, *self-sacrificing love.
- 1897 Gladstone E. Crisis 14 ― In the midst of a high and self-sacrificing enthusiasm, the Greek Government and people have shown good sense.
- 1882 Advance (Chicago) 5 Oct., ― *Self-sacrificingly non-denominational in all directions.
- 1871 Smiles Charac. ix. (1876) 239 ― In *self-sacrificingness,··in the ordinary intercourse of life, mainly consists the difference between being well and ill bred.
You might be looking for a word martyr here:
a person who suffers very much or is killed because of their religious or political beliefs , and is often admired because of it:
a Christian / Islamic / religious martyr
She fought against racism all her life and died a martyr to the cause
Other words could be: a war hero, man of courage, man of the hour, conquering hero, ,lionheart (as in Braveheart film. The hero dies at the end of the film)