In the Evangelical minds: Those fetuses may be white fetuses and could become Christians, god has a positive plan for them, those Palestinians are heathens who work in service to a false religion and are defying god's plan for establishing 'israel' and bringing about the end times. As in their holy book their god has commanded the massacre by certified (by god) 'good guys' of men, women, even children and in fact done so himself with a great genocidal flood these Palestinians are disposable because they are in the way of the holy plan and in fact have stolen what god gave to the ancient Jews. In their minds the Palestinians should have simply left, gotten out of the way, they believe instead they are fighting the holy plan, the holy claims to land, the process and thus god himself.

It's divine command morality. That is that which god commands is good. God can make rules like murder is bad BUT god can also carve out exceptions to those rules and this is one of those exceptions. So they know they'll see their heaven because they're doing violence for their god, they're willing to kill for their god. They're willing to kill to ensure the end times comes and their god's plan comes to fruition.

It doesn't have to follow some ironclad non-divine logic like utilitarianism or something, it just has to follow the basic rules or exist in one of the many wide carve-outs to those rules and their interpretations.

At the end of the day they have a line which hand-waves away any problems with the process itself and that is that god works in mysterious ways.

In this case those mysterious ways are having occupiers butcher children with clear evidence of atrocities that go way beyond collateral damage. They believe for example in a story of a king, I want to say perhaps David but I'm possibly mixing it up, this king was not a good and obedient Jew or Christian but he did serve god's purposes in a very realpolitik way so was elevated by god and commanded to be celebrated by god despite him doing sinful things on the side because he served the greater purpose. And these Palestinians being murdered also serves that greater purpose. And there is a tale in The Bible of god's army defeating an enemy people and god commanding them to kill all the men and boys but to keep the girls alive for their own uses (yes that's exactly what it sounds like). Anyone who can look at that and say that's a basis for morality should have zero issue excusing this.

It's perfectly logically consistent within their worldview, you can't force outside morality on it except by incredible violence and threat and really the only way to tear it down is to poke holes in the underlying ideology, attack the basis of their belief-system and undermine it which takes way more than a couple of conversations, I'm talking months, years of patient effort back and forth, reading their arguments and responding to them and in many cases they at some point dip out anyways and refuse to engage further in logical argument meaning you may have wasted months on them. They're lead by people fully invested in this way of thinking and have been since the end of WW2.