Mass executions were and still
are commonplace in the Middle East, and of course also apply to the
Divinely authorised actions of Moses, Joshua, Samuel, Saul and the
judges as much as to David.
Precisely the same penalty was applied to Israel itself when it fell
into sin, as it will be for us and our cities under an even greater
burden of sin now. Sodom and Gomorrah, Admah and Zeboim are set forth as
patterns of judgement, and our condemnation for sinning against light
is likely, like Capernaum and Bethsaida, to be greater.
As to the issue of torture, I agree that your interpretation is plausible, it agrees with the Septuagint and that many older
Christian commentators hold it, whilst blaming David.
It is noteworthy it took place at the time of David's greatest dual
sins of murder and adultery, and betrays an episode of unique
callousness in Israel's history.
The word used for cut is unique and derives from שׂור which can also
mean 'to rule, contend, have power, prevail over'. Of the 24 Bible uses
of הֶעֱבִיר to cause to pass, only 3 others have evil sense of killing
by passing through a fire, the first two examples are of separating to
or dedicating to a specific function. Whilst it is a minority position,
this may mean David's act was to separate the Ammonites to humiliating
forced labour as some modern translations like the NIV have assumed. 'Consigning
them to labor with saws and with iron picks and axes, and he made them
work at brickmaking.'
Torture was commonplace in the region, see Amos.1.3 and Ammon's
involvement in brutalities in Amos 1.13, which are indeed treated as
crimes. The brick kilns emulate the sin of Malcon (מלכן or Molech) and
which the Ammonites like many others inflicted on their children. 2
Kings 16.3; Lev.18.21,27; 20.2; Deut. 18.10. This would not justify
David's action, [indeed we consider it reprehensible]. He was indeed a man of blood (2Sa 16:8, 1Ch 28:3), as
indeed will be the avenging Messiah, Whom David depicts at His return
(Isa.63.1-3, Rev.14.20, 1 Thess.1.7-8), though this time in perfect and
unimpeachable justice, which is His right alone.
However
there are strong Biblical grounds to doubt that David's armies did
torture the Ammonites, even though it was plain that they had engaged
in mass executions for example of the Moabites and others before as
you've mentioned (2 Sa 8:2), sometimes even after a victory was
concluded.
First there is the practical issue of
using harrows to cut people with. It is hardly an efficient means of
execution or of torture. What would the armies do, ask the Ammonites to
lie in the field and then treat them like a crop? When one studies an
image of a harrow, it looks clumsy and inefficient.

Secondly,
there was the reputation of Israelite kings, even in the apostate
Northern Kingdom, when Benhadad was cornered by a man he had recently
grievously insulted, he knew that Israeli kings had a soft spot for
mercy and compassion, 'we have heard that the kings of the house of
Israel are merciful kings'(1 Ki 20.31). Where did he get this idea
from, especially if the greatest king of the undivided Kingdom was the
greatest war criminal in history, as you've depicted him? Did Ahab have
such an impressive personal reputation for clemency, after slaughtering
hundreds of his own citizens?
Thirdly, there is a very
remarkable event that takes place during David's escape from his
murderous son Absalom. At a time when David's situation is at
its lowest, when his enemies are revealing themselves, when one
enemy feels free to stone and curse him, a party of loyalists show
remarkable loyalty and affection for him. They could not have done so
for personal advantage, for Absalom might well have been expected to
wreak revenge, but out of a sense of debt and deep compassion.
Out
of this party is a remarkable figure, Shobi the son of Nahash of Rabbah
of the children of Ammon (2 Sam 17.27-8). Along with the gracious
Barzillai the Gileadite, they bring the king, 'beds, and basons, and
earthen vessels, and wheat, and barley, and flour, and parched corn,
and beans, and lentiles, and parched pulse, and honey, and butter, and
sheep, and cheese of kine, for David, and for the people that were with
him, to eat: for they said, The people is hungry, and weary, and
thirsty, in the wilderness'. Lavish provision then - but this Shobi is
none other than the son of the Nahash who insulted David in the first
place, and whose generation suffered the things described in the text
at hand (1 Chron. 20.3). How could this be? Even if he was as seems
likely now a Jewish proselyte, would Shobi have forgiven and forgotten
such brutal atrocities against his own family and his own people? It
seems very unlikely.
For once, I think the NIV is right.