The idea that one particular party is owed my alliance no matter what atrocities they support is fundamentally authoritarian which in-itself is a principle I do not abide by.
Given your logic, all is permissible so long as we have one party we can point to and say "they're worse". Which is what leads to as situations where both parties continually move away from the interest of the people. I have far more respect for people who deny there is a genocide, as opposed to those that see it and actively choose to ignore it.
Also what is "would be"? Harris and Biden have been presiding over and supporting the current genocide. I have a hard time imagining it being truly worse during a Trump administration. At the very least mainstream Democrats can allow themselves to oppose genocide now that it no longer interferes with the aims of their party.
It continues to boggle my mind that liberal Democratic will endorse and support genocide without a moment's hesitation or reflection.
Trump's encouragement to 'let Israel finish the job' seems worse.
In a way it's like a significant number of Americans feel the same way. The people who elected Trump, then Biden, then Trump again.
As someone who leans quite left (and voted 3rd party in a deep-blue state), I can completely understand why many traditional Democratic voters didn't turn out (and why many Republicans despise the Democrats enough to presumably vote against their interests as you pose).
The largest issue for me is that I cannot support genocide. The "I'm speaking" (to protesters) was repulsive. The culture of "if you don't get on board it's your fault if democracy dies" attitude of the Democratic party was just as fascist sounding to my ears as anything they claimed the Republicans have in store for the future. I personally can't fathom how any person that aligns with my view of the world would basically take the stance of "genocide doesn't matter, toe the line". For me personally, two parties that aggressively support continue apartheid conditions and genocide are both against my interest so profoundly that where they differ on issues is irrelevant.
Furthermore the Democratic party has increasingly come to represent a very anti-democratic institution. Biden was promised us as a one-term president to get things patched over while new leadership was established. Then the fact that he was clearly increasingly incompetent was hidden until it was embarrassingly too late. But oddly, it was not too late for a primary, where Democrats could choose a candidate, but we didn't get that. And yet, when mentioning any issues with the mass murder of children you are told to "shut up and get in line".
Finally, Biden didn't deliver on any of the meaningful promises he made. Nothing happened to improve abortion issues while he was president, his track record on climate was just as meaningless and awful as any Republican, children still sat in US detention centers separated from their parents, corporate interests still take precedence over the rights of workers just as much as with any Republican.
While I am nervous about another Trump term, I fail to see how the world was so much brighter under Biden. The Democrats have become the party of "shut up and do as we say because we know better" with no objective improvements in the issues I care about when they are in office, which is impossible for me to get behind.