Either
Suppose you were living several thousand years ago in a village in the Near East. Put yourself in that context.
You receive word from a messenger that a nearby village has recently been destroyed by another tribe who claims that they were told by their god that they are chosen people. The tribe is on their way and were told by their god that they must "totally destroy all that belongs to you". Furthermore they were told by their god to "not spare you; put to death your men and women, children and infants, cattle and sheep, camels and donkeys." The messenger also says that these people say that anyone who believes in a different god must not be spared, should be put to the sword and killed.
Now suppose, you are living today in Syria. Put yourself in that context. Suppose you hear the same thing about a group called ISIS.
Same region of the world. Different time. If you could time travel back and forth between the two time periods, is there any method that you, as an external observer, could use to ascertain that the God of the Israelites should be worshipped while the God of the Muslims should be fought against?
Note 1: Remember to picture this from the perspective of an ancient Near Easterner. You do not yet have the hindsight view of post-New Testament interpretations. In the same way, today we do not have the potential hindsight view of ISIS in the year 3000 A.D.
Note 2: For reference I am looking at 1 Samuel 15, Deuteronomy 13, and Numbers 31 with regards to Christian beliefs and Quran 2:191, Quran 9:123, Quran 9:5, among others with regards to the beliefs of Muslim extremists (e.g. ISIS).
Just read the OP again, I’m not completely sure if I’ve understood what you are asking but I’m assuming that you’re not questioning along the lines of ‘what’s the difference between being killed by this group or that group?’, i.e a soldier killed by the enemy or by friendly fire is dead either way, so no difference there. I’m assuming you’re asking a qualitative question about the motivations people carrying out the killing.
To answer that I think you have to take several steps back and review the context on different levels, broadly, in terms of the nature of reality as we experience it, the context of the time and place, and the eventual purpose of the tribe of Israel in history. To understand
that requires a lot more time and I don’t think that can be easily addressed in a short post, at least I can’t think of a way that could be done. What you’re looking at there are the differences between a world dominated by the worldview of a disturbing culture like that of the Amalekites and that which we have actually inherited - it takes a lot of careful investigation to put it together, as it is far from a linear progression and so easy to pick holes in from superficial perspectives, but, contrary to popular modern beliefs, the Judeo-Christian influence on modern Western society is what we ultimately have to thank for a lot of the individual freedoms we enjoy. Answers that don’t address these questions of context ultimately come down to pointless virtue signalling and random assertions that add nothing to understanding.
To get to grips with the ‘nature of reality’ from a biblical perspective you have to ask questions like ‘what is sin’ ‘why is it so destructive’ ‘what is free will’, ‘
why does sin lead to death?’ and so on, you need to have an understanding of those issues in order to begin to understand the sometimes violent struggle for Israel’s physical survival and spiritual identity. To my mind Nicolás Gómez Dávila sums this way of thinking up effectively -
‘No (moral) limit is inherent to a being; no ambition denies itself. Every denial is born of an obstacle, all abstention of a rejection. The universe is a system of reciprocal limitations, where the object is constructed as a state of tension between conflicting forces. Violence, the cruel minister of the limited nature of things, imposes the standards of actualised existence’.
Bearing in mind that David/Saul didn’t actually carry out ‘genocide’ and weren’t the initial aggressors or invaders - which is a very simplified idea of what happened - (read all of 1 Sam 27-end) Questions about the survival of the Amalekites vs the survival of Israel are either/or questions. After the 400 years between God’s stated intent (to Abraham) to judge the Amalekites and the eventual execution of this judgement, they made themselves intolerable in a universe in which salvation could only remain a possibility through the survival of the Jewish nation. That is the brutal nature of it. We see that every day - sin has devastating affects. A person who is abused as a child will often be targeted by predators for the rest of their life due to the affects of the original abuse, the affects of neglectful or abusive parents, beliefs in caste systems, etc etc etc have real life, devastating, terrible effects. That is reality.
The Israelites were not an invading force, they were fighting for their own survival. Usually this fight had limitations of necessity, in some cases it involved eliminating the threat (although not actually completely as it happens). ISIS is/was an invading force, actively invading territories and subjecting the peoples therein to their own interpretations of sharia law. This is the nature of Islam from the beginning - as Bertrand Russell summed up, to all intents and purposes in any of it’s expansionary phases it was (for the leadership) a religion of convenience used for political and economic ends (of course Christianity has been bent to that end at times but an exploration of the whys and wherefores might be an subject for another post - the essential differences are in does the text of the religion teach what it is being used for?), through violence. Christians and Jews living under Muslim occupation are (if the law is enforced) relegated to an impossible state of complete non self expression on pain of death, such impossible limits are placed on their behaviour by Islamic law that there is no alternative but to convert or die. Israel was, as far as possible without leaving itself open to complete destruction, a community of open borders to strangers and the oppressed, living wherever possible in peace with it’s neighbours. But to understand that be prepared for a lot of study, to get past the popular misconceptions based on selected verses.
Beyond
that an important question to ask is what is being presented in the accounts of a particular person’s behaviour, e.g. David? Biblical heroes are presented ‘warts and all’, with all of their mistakes, personal sins and so on. Sometimes people think that if a biblical character behaves in such and such a way, then that means his or her behaviour is being held up as a good example, which is far from being true. To understand the behaviour of biblical characters held up as ‘heroes of faith’ it is essential to have some grasp of their life and times, what was considered normal, what was simply expedient, what their ultimate ends were and so on. Trying to understand the actions of, for example, kind David from a purely 21stC perspective is pointless. Looked at as a whole, with the large timescales, while God may be deeply involved in human affairs in some sense he also refrains from intruding except very rarely. He places a great deal of responsibility on our shoulders, and provides us with the means to understand the very real outcomes of our choices and actions, giving us the space to choose and act as we see fit.