Group think and social pressures. It is remarkably easy for humans to find themselves behaving like those around them. The progression doesn't even have to be fast. Imagine that when the israeli military fought wars in the mid 20th century, they started thinking of the enemy around them, and then imagine how long it has been since then with no corrective measures ever taken on those lines of thinking. NOW add in that israel has mandatory military service. The ones who choose to stay are going to be the ones that can agree and fit in with those now very evolved thoughts of hatred, and of course they will bring their own worse additions as they try to prove themselves to those the esteem, the already putrid military members ranked above them.