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.