Why such harsh treatment of the enemies of Israel in the Old Testament?
Because grace is not ordinarily “sensible” but supernatural, it is easy to underestimate the fact that the world is quite different now because of the work of Jesus who is True God taken flesh upon Himself (and thus is also True man).
The Israelites initially were asked to show their enemies “no mercy”.
WHAT was “utterly destroy them” and “no mercy” initially? And WHY were the Israelites to show “no mercy” to their enemies?
“Utterly destroy them” and show them “no mercy” initially merely meant things such as don’t make covenants or intermarry with them or allow pagan worship to be carried out.
Because the grace was not yet afforded to the world from Jesus work on Calvary at that time. Thus for Israel to consort with the pagans, the paganism would . . . . rub off on them.
Case in point with Egypt and the Golden Calf earlier.
Let’s look at this again.
WHAT was (initially) “utterly destroy them” and show “no mercy”? And WHY were the Israelites to show “no mercy” to their enemies?
Initially “no mercy” was:
DEUTERONOMY 7: 1-2a,c 1 "When the LORD your God brings you into the land which you are entering to take possession of it, and clears away many nations before you, the Hittites, the Girgashites, the Amorites, the Canaanites, the Perizzites, the Hivites, and the Jebusites, seven nations greater and mightier than yourselves, 2 and when the LORD your God gives them over to you, and you defeat them; then you must utterly destroy them . . . . and show no mercy to them. . .
Well this meant to kill them all right?
What did “utterly destroy them” . . . . and “show no mercy to them” mean at this point in time then?
Fortunately Deuteronomy 7 tells us.
DEUTERONOMY 7:2-3, 5 2 and when the LORD your God gives them over to you, and you defeat them; then you must utterly destroy them; you shall make no covenant with them, and show no mercy to them. 3 You shall not make marriages with them, giving your daughters to their sons or taking their daughters for your sons. . . . 5 But thus shall you deal with them: you shall break down their altars, and dash in pieces their pillars, and hew down their Asherim, and burn their graven images with fire.
**Show no mercy and utterly destroy in this context means . . . **
*]Make no covenant with them
*]Not intermarry with them
*]Dispossess the pagan worship accoutrements
WHY? Why should the Israelites not make Covenant with them or marriages?
Again fortunately Deuteronomy 7 tells us in the very next verse.
DEUTERONOMY 7:4 4 For they would turn away your sons from following me, to serve other gods; then the anger of the LORD would be kindled against you, and he would destroy you quickly.
Later as Israel again soaks in paganism (as they had in Egypt) such as having illicit sexual relations with the pagans (i.e. part of Balaam’s error), and eventually sacrificing their own children to phony pagan gods, now the remediation becomes more fatal to the pagans.
Partly because of the pagans sins themselves, but ALSO in part due to Israel’s sins.
This radical action was meant to help preserve at least some of Israel.
Later, the context of “show no mercy” and “utterly destroy” changes.
Not because God changed. But because the Israelites changed (dipped themselves in more sinful pagan practices).
This is part (not all) of what Ezekiel talks of in Ezekiel 16 for example.
EZEKIEL 16:26-37 26 You also played the harlot with the Egyptians, your lustful neighbors, multiplying your harlotry, to provoke me to anger. 27 Behold, therefore, I stretched out my hand against you, and diminished your allotted portion, and delivered you to the greed of your enemies, the daughters of the Philistines, who were ashamed of your lewd behavior. 28 You played the harlot also with the Assyrians, because you were insatiable; yea, you played the harlot with them, and still you were not satisfied. 29 You multiplied your harlotry also with the trading land of Chaldea; and even with this you were not satisfied. 30 "How lovesick is your heart, says the Lord GOD, seeing you did all these things, the deeds of a brazen harlot; 31 building your vaulted chamber at the head of every street, and making your lofty place in every square. Yet you were not like a harlot, because you scorned hire. 32 Adulterous wife, who receives strangers instead of her husband! 33 Men give gifts to all harlots; but you gave your gifts to all your lovers, bribing them to come to you from every side for your harlotries. 34 So you were different from other women in your harlotries: none solicited you to play the harlot; and you gave hire, while no hire was given to you; therefore you were different. 35 "Wherefore, O harlot, hear the word of the LORD: 36 Thus says the Lord GOD, Because your shame was laid bare and your nakedness uncovered in your harlotries with your lovers, and because of all your idols, and because of the blood of your children that you gave to them, 37 therefore, behold, I will gather all your lovers, with whom you took pleasure, all those you loved and all those you loathed; I will gather them against you from every side, and will uncover your nakedness to them, that they may see all your nakedness.
In the New Covenant with the work of Christ, the world (most especially Christians) has graces to rise above sin that were not available in the Old Covenant days. That’s WHY we do not carry out such harsh treatment of enemies in the New Covenant.
The New Covenant standard is Jesus Christ.