Why are so many Pro-Palestine (I am pro-Palestine and anti-genocide) Americans refusing to vote for Harris due to her stance on Israel?

There is quite a lot Trump could do to speed things up. He could, for one thing, send American troops to assist Israel on the ground; I have concern that he might do such, because Israel has increasingly been dragging other countries in the region into this, notably Iran, and Trump pursued a policy towards that country during his term in office that very well could have led to war had things gone slightly worse. Given his support for Netanyahu, whose government has itself been tempting fate of late by engaging in back and forth missile strikes, and his disregard for the consequences of attacks against Iran, I have serious fears that he might give Israel a green light to pursue a full scale war with that country, by promising to commit US forces in the event of such a thing.

At a lesser degree, he also could simply increase US military aid to Israel beyond the current level, and end what efforts (insufficient by a country mile but still better than their absence would be) have been made by the US to convince Israel to limit its actions, such as the recent threats to cut some of its military aid if Israel does not allow more food aid across Gaza. He appears to actively dislike Muslim populations, as seen by his efforts as president to ban travelers from Muslim majority countries, so it strikes me as rather unlikely that he would do anything, even something basic like that, to assist a Muslim majority country like Palestine against the wishes of one of his allies.

Also for the record, I do not think that I am simply protecting "my outgroup" in opposing him. I am of the view that he, (or more importantly, the fascistic movement that he has grown around him, of which Trump himself is the leader, but which may persist even after he is gone), presents an existential threat not just to myself and those whom I know, but to you, to everyone in the country, to everyone in the numerous countries who he seems actively hostile to (including but not limited to Palestine as I have said, and Iran, as I was saying earlier, and Ukraine), and to a lesser extent, to the future of every single person on this planet. That may sound a bit extreme, but we are talking about making a narcissistic old man showing signs of mental decline and known for lashing out at things that anger him the commander in chief of a nuclear armed state, we are talking about putting someone who does not seem to believe in climate change at the head of the world's largest economy at a time when getting carbon emissions down is critical to keeping the planet livable in the future, and we are talking about putting the country with the world's largest military budget in the hands of a person who idealizes fascists, has attempted to maintain power despite a previous election loss, and has a following composed to a large degree of racists and religious zealots.

I am not saying that I worry about what Trump will do as hyperbole, or to justify what the current dem administration has done in arming Israel while it bombs and shoots civilians, I am saying that I worry about what he will do, because thinking about it quite literally keeps me up at night and has quite literally given me actual panic attacks within recent weeks upon seeing the prevalence of his support in polls and among my coworkers.

I do not think the democrats are actually "willing to throw out trans people" the way you seem to suggest at the end there. I dont even think that they are happy with what their "ally" in Israel is doing. I think they are a fragile "everything that isn't the R's" alliance of much of the right and what passes for the left here that includes both LGBT people and their allies, and conservative types who never wanted them in their party in the first place but arent quite extreme enough for the republicans, who are sort of mashed together in a broad coalition that as a result has no real collectively agreed upon ideology and doesn't have the guts to rock the boat by withholding military aid to a country traditionally seen as an ally, even though that country really deserves to have that aid cut right now. Their vague compromises of positions do not really align with mine on many if not most things, especially economic and foreign policy, and I resent that they stay just barely to the left of the republicans to get the support of the left while offering it little but scraps. I do not like them, except maybe a few on the leftmost edge. But we (or at least I, I guess I've just assumed you were probably also American if youre invested in our election but I guess with our international influence that doesnt actually mean much) live under a system that guarantees that if they dont win, Trump will, and when he and his cult look so startlingly similar to the fascists of history, just before they succeed in subverting the systems that constrained them, not voting for them is a luxury that I do not think that I or any of us in this country really have.