what degree of hatred of "the enemy" needs to be installed in young army men (& not just the Israelis, the US & other forces in Iraq, Vietnam, etc, etc,...) to make them so insensitive to killing people they don't even know?
Hatred of "the other" has often been used as a manipulative tool in warfare, dating just about as far back as there are historical records. At the same time, state policy can modify or even eliminate this. In his Discourses on the First Ten Books of Titus Livius
, Nicolo Machiavelli outlines the policy of the Roman Republic, which had a gradation of responses to their enemies, based on the degree of resistance in arms the people made to them. Generally, though, war in ancient time was a bad business for all concerned, as few armies exercised the kind of discipline which allowed Rome to conquer their corner of the world.
In Europe of the middle ages, total warfare was considered very bad form, and there were numerous instances of people who had behaved "nobly" being spared, and even more unusual (to our eyes) circumstances prevailed. People captured in battle who could bring a high ransom were often paroled to go home to arrange for the collection of the ransom (paroled meaning simply that they gave their word), who returned to their captors voluntarily, it being a matter which touched their honor. The wars of the Protestant Reformation had involved many incidents of brutality, to the extent that Europe universally condemned it. It was recognized that the excesses of the followers of an army could be mitigated, it not completely controlled. Wallenstein's army during the Thirty Years War numbered about 40,000 men when he marched across northern Germany, but probably mounted to 100,000 or more counting all the camp followers and scavengers in his wake. He did nothing to control them or to police his troops and their hangers-on. At the same time, the Imperialist general Count Tilly exercised an iron discipline over his men, and looters and rapists were executed out of hand if captured by his field police (most generals of the day didn't even bother with field police)--yet he was so respected by his men that they called him "Father Tilly." King Gustav Adolf of Sweden exercised the same kind of discipline over his Swedes, and the Germans taken into service in the Swedish army. Shallow observers have pointed out that Swedes were tried in German cities and towns for their excesses, but more perceptive historians have pointed out that no other army of the day allowed local authorities to bring their men to trial.
Especially after the Thiry Years War, the European nations condemned the use of what was then known as "terror," and sought to conduct wars in a civilized manner. When in the War of the Spanish succession, in 1704, Marlborough and Prince Eugene of Savoy burned large parts of Bavaria (carefully sparing the estates of Max Emmanuel of Bavaria, the Prince Elector who had betrayed the Emperor and gone over to the French)--they were widely condemned for their uncivilized practice.
In those days, armies were "professional" to the extent that they were all either hired as individuals, or hired from a local prince or magnate who sold the services of his or her troops. So, for example, the dowager "princess" of one of the Hesse's which disappeared in the Thirty Years War tried to keep the inheritance of her son alive by hiring out her troops to the highest bidder, without reference to allegiance. The tradition continued, which is why any German mercenaries in the American Revolution were roundly condemned and called "Hessians," whether they came from one of the Hesses or not. The "Great Elector" of Brandenburg, Friedrich Wilhelm, used the "leasing" of his army as a means of building up the poor finances of a basically indefensible state (what we would think of as Prussia) in order to provide the wherewithal to support a large army on slim means.
It was not until the French Revolution that the massive levies of men of military age created the huge armies that we associate with modern warfare. Just as in the Thirty Years War, the tone was set by the commanders. Many of the commanders during the Wars of the French Revolution took the attitude that they were "liberating" other parts of Europe, and that their men must be held in check--they were not popular with their governments. Napoleon, however, when he invaded Italy for the first time in 1796, looted with genuine abandon, delighting his masters in the government of the Directory, disabusing the Italians of any illusions about liberation they may have had, and gaining the love of his soldiers, who had previously been unpaid, often unshod and clothed in rags. This is not to say that he cared much about his men--the medical services in his armies were appallingly bad, and this despite the fact that the French had pioneered military surgery and medical care under Louis XIV. Napoleon only cared about himself and his family, but he usually won, so the men loved him anyway--soldiers love a winner, no matter how he treats them.
Throughout most of history, intelligent commanders and governments have recognized that dead peasants don't pay tribute, and this, if nothing else has usually lead them to restrain their troops. The European reaction to the religious wars of the 16th and 17th century was largely conditioned by a recognition that the destruction of the peasants and the merchants in the towns destroyed the means by which they could hire and pay their soldiers, and destroyed the tax base for years to come. That may seem cynical, but then, successful soldiers need to be pragmatists. Even the dreaded Mongol horde would hold their collective hand if the local authorities coughed up the tribute right away--Temujin was smart enough to know upon which side his bread was buttered.
Encouraging or tolerating hatred does violence to our modern values, as well as doing violence directly to the victims. The effect of war on the soldiers has reasonably been compared to rape. Any state which tolerates or encourages brutality is ultimately going to isolate itself. Whether or not that will be a bad thing, though, so often depends upon circumstanced. The Romans and the Mongols could be as brutal as they chose--no one was going to stop them. It might be a bad idea, though, for the Israeli state.