Since the written text in use at the time lacked the vowels found in the Masoretic Text (MT), the issue is primarily over how the consonants דבר are pronounced, either as דָּבָר (dabar) = speech, word, speaking, thing or as דֶּבֶר (deber) = pestilence, plague. According to the lexicon a proper word for dabar would be λόγος (logos) or ῥῆμα (rhema) or λόγιον (logion) and deber would be λοιμός (loimos). The consonants appear 47 times in Isaiah and the MT has all 47 as dabar. The LXX translates 31 as logos, 9 as rhema, and 1 as logion. Of the remaining 6, only the use at 9:8 is considered as deber. Given the overall handling of the consonants the one exception appears to be intentional.
There is also a question on how to understand the action described:
The Lord has sent a word against Jacob, and it will fall on Israel (ESV)
My Lord d-let loose a word-d against Jacob and it fell upon Israel. (Isaiah 9:7 JPS)
d-d Septuagint reads "Let loose pestilence"; cf. Amos 4.10. In vv. 7-20 Isaiah alludes to and builds upon Amos 4.10-12
In his comments on Isaiah, Benjamin D. Sommer explains this issue:
The verbs here are in the past tense, but their significance is unclear. They may predict disasters to come (in which case the verbs exemplify "the prophetic past" described in 9.1-6n.); alternatively they may review disasters that God already sent in an unsuccessful attempt to chasten the Northern Kingdom (in which case the prophet does not predict the coming events but presents an interpretation of recent history).
How Does the LXX Understand דֶּבֶר deber?
Using the KJV, דֶּבֶר (deber) is found 49 times in 14 different OT books:
Verse Heb Greek Verse Heb Greek Verse Heb Greek
Ex 5:3 בַּדֶּ֖בֶר θανατος Jer 14:12 וּבַדֶּ֔בֶר θανατω Jer 44:13 וּבַדָּֽבֶר θανατω
Ex 9:3 דֶּ֖בֶר θανατος Jer 21:6 בְּדֶ֥בֶר θανατω Ez 5:12 בַּדֶּ֣בֶר θανατω
Ex 9:15 בַּדָּ֑בֶר θανατω Jer 21:7 הַדֶּ֣בֶר θανατου Ez 5:17 וְדֶ֥בֶר θανατος
Lev 26:25 דֶ֙בֶר֙ θανατον Jer 21:9 וּבַדָּ֑בֶר --- Ez 6:11 וּבַדֶּ֖בֶר θανατω
Num 14:12 בַדֶּ֖בֶר θανατω Jer 24:10 הַדָּ֑בֶר θανατον Ez 6:12 בַּדֶּ֣בֶר θανατω
Deut 28:21 הַדָּ֑בֶר θανατον Jer 27:8 וּבַדֶּ֜בֶר λοιμω (x) Ez 7:15 וְהַדֶּ֥בֶר θανατος
2 Sa 24:13 דֶּ֙בֶר֙ θανατον Jer 27:13 וּבַדָּ֑בֶר֙ λοιμω (x) Ez 7:15 וָדֶ֖בֶר θανατος
2 Sa 24:15 דֶּ֙בֶר֙ θανατον Jer 28:8 וּלְדָֽבֶר λοιμου (x) Ez 12:16 וּמִדָּ֑בֶר θανατου
1 Ki 8:37 דֶּ֣בֶר θανατος Jer 29:17 הַדָּ֑בֶר λοιμον (x) Ez 14:19 דֶּ֥בֶר θανατον
1 Ch 21:12 וְדֶ֙בֶר֙ θανατον Jer 29:18 וּבַדָּ֑בֶר λοιμω (x) Ez 14:21 וָדֶ֔בֶר θανατον
1 Ch 21:14 דֶּ֖בֶר θανατον Jer 32:24 וְהַדָּ֑בֶר λοιμου (x) Ez 28:23 דֶּ֤בֶר θανατος
2 Ch 6:28 דֶּ֣בֶר θανατος Jer 32:36 וּבַדָּֽבֶר λοιμω (x) Ez 33:27 בַּדֶּ֥בֶר θανατω
2 Ch 7:13 דֶּ֖בֶר θανατον Jer 34:17 הַדֶּ֣בֶר θανατον Ez 38:22 בְּדֶ֣בֶר θανατω
2 Ch 20:9 וְדֶ֣בֶר θανατος Jer 38:2 וּבַדָּ֑בֶר λοιμω (x) Hos 13:14 דְבָרֶיךָ θανατον
Ps 78:50 לַדֶּ֥בֶר θανατον Jer 42:17 וּבַדָּ֑בֶר λοιμω (x) Am 4:10 דֶּ֙בֶר֙ θανατον
Ps 91:3 מִדֶּ֥בֶר λογου Jer 42:22 וּבַדֶּ֖בֶר λοιμω (x) Hab 3:5 דָּ֑בֶר λογος
Ps 91:6 מִ֭דֶּבֶר πραγματος (x) - some manuscripts
θάνατος (thanatos 2288) is the most common choice (34 times, 69%) to translate deber. Twice the consonants are treated as dabar and translated as λόγος (logos) 3056; once as πρᾶγμα (pragma 4229) and 10 times, all in Jeremiah it is translated as λοιμός (loimos 3061). .
The only book where deber is treated differently is Jeremiah where only 38% are translated as θάνατος. There are wide divergences between the LXX and MT in Jeremiah.
2 If these are discounted, θάνατος is used 88% of the time (28 of 32). Also, 2 of the 4 exceptions treat the Hebrew as דָּבָר (dabar), the other possible pronunciation.
Some translations (i.e. JPS) note "pestilence" is a possible meaning for θάνατος. That does not appear to be how the LXX understands the word. One example:
He made a path for his anger;
he did not spare them from death (מִמָּ֣וֶת),
but gave their lives over to the plague (לַדֶּ֥בֶר). (Psalm 78:50 ESV)
He made a path for his wrath; he did not spare their souls from death, and their cattle he consigned to death. (Psalm 77(78):50 LXX NET)
Here the LXX also renders מָוֶת meaning death as θανατον. Therefore, when the LXX translator considers דבר as דֶּבֶר deber, the reader should expect to find θανατον meaning "death."
Isaiah 9:8(7) and Amos 4:10
The Lord sent death against Iakob, and it came on Israel. (Isaiah 9:8(7) NETS)
Θάνατον ἀπέστειλεν κύριος ἐπὶ Ιακωβ, καὶ ἦλθεν ἐπὶ Ισραηλ (LXX)
The verse is correctly placed in the past tense. If דבר were to be translated as "word" the reader would understand the divine word was sent against Jacob and it fell only on Israel. For the LXX translator, it would make little sense to say the word sent to Jacob fell only on Israel (the Northern Kingdom). Nor would it be historically accurate: the word went to both kingdoms.
The translator knows all warnings to both the Northern and Southern Kingdoms were ignored. They are also living at a time in which the Southern Kingdom had experienced restoration. The LORD had dealt with both but Israel never had an Ezra or Nehemiah to begin to put things back in order. Israel was "dead" because of what the LORD had done. For one considering the passage as recent history, deber makes more sense than dabar and it would be translated as θάνατος.
Moreover, the LXX is historically accurate. The LORD sent death against Jacob (both the Northern and Southern Kingdoms) but, as the Southern Kingdom had been revived, death fell only on Israel.
It is obvious, as Sommer notes is possible, the LXX translator decided to convey this passage as an interpretation of history not a prediction of disasters to come. Accordingly the translator understands דבר as דֶּבֶר deber and renders it as θάνατος. The LXX correctly informs the reader the Northern Kingdom (only) is dead because of what the LORD had done. In fact, historically neither "word" nor "pestilence" would accurately convey the divine action taken against the Northern Kingdom.
The JPS translator also notes a connection between this passage in Isaiah and Amos 4:10-12:
I sent you death in the way of Egypt, and I killed your young men with the sword, along with the captivity of your horses, and I brought up your camps with fire in your wrath; even so you did not return to me, says the Lord. I overthrew you as God overthrew Sodoma and Gomorra, and you were like a firebrand snatched from the fire; even so you did not return to me, says the Lord. Therefore, thus I will do to you, O Israel, but because I will do thus to you, prepare to call upon your God, O Israel. (Amos 4:10-12 NETS)
Like Isaiah 9:8(7) the LXX has θανατον "death" for the Hebrew דֶּבֶר deber in Amos 4:10, indicating a consistent handling of the consonants in the message.
The LXX understands the prophet has interpreted recent history and so conveys their understanding of that history to their reader. It is a fact the LORD who tore the Kingdom in two to make Israel, later put Israel to death. Significantly, there is no reason for the LXX translator to believe the future will once again be made up of a divided kingdom and there is no reason not to say death fell on Israel.
1. Benjamin D. Sommer, The Jewish Study Bible, Oxford University Press, 2007, p. 802
2. How do the traditions of the LXX and MT versions of Jeremiah relate?