Three points:

• Biden and Harris are right now with their actions physically supporting the Genocide. Trump talks about supporting the Genocide even more. Well, guess what: Trump lies shamelessly (as the Democrat propaganda here doesn't stop reminding us of in everything but, "strangely", not this subject) and isn't even competent when it comes to actual execution. So on one side we have an absolute certainty that the candidate supports the Genocide and on the other one we have a probability that its so based on the statements of a known liar. I would say the claims that Trump is worse on this are doing a lot of relying on Trump's word (on this subject alone) in order to elevate his evilness of this above that of people who are actually, right now, shamelessly and unwaveringly supporting the Genocide with actual actions.
• If the Leadership of Democrat Party manages to whilst refusing to walk back on their active support of a Genocide, win the election with a "otherwise it's Trump" strategy, they will move even further to the Right because that confirms to them that they can do whatever they want and still keep in power. Now, keep in mind that the Democract Party leadership already supports Fascism (ethno-Fascism, even, which is the same kind as the Nazis practiced), so far only abroad (whilst Trump does support Fascism at home) so there isn't much more to the Right of that before Fascism at home. You see, for a Leftie voting Democrat now will probably be the least bad option in the short term, but it's very likely to be the worst option in the long term because it consolidates and even accelerates the move of the Democrat Party to the Right.
• Some people simply put their moral principles above "yeah but" excuses and won't vote for people supporting the mass murder of children.

In summary:

• Trump's Genocide support is a probability based on his word, willingness and ability to fulfill it (i.e. his competence at doing it), whilst Harris' is an actual proven fact with actions happening right now.
• A vote for the Democrats whilst their policies are so far to the Right that they're supporting modern Nazis with the very weapons they use to mass murder civilians of the "wrong" ethnicity, if it leads to a Harris victory will consolidate this de facto Far-Right status of the party and maintain momentum in going Rightwards. Voting like that is, IMHO, a Strategically stupid choice even if the case can be made (and that's the entirety of what the Democrat propaganda here does) that Tactically it's the least bad choice.
• Some people can't just swallow their moral principles, especially for making a choice which isn't even a "choose a good thing" but actually a "choose a lesser evil", and "Thou shall not mass murder thousands of babies" is pretty strong as moral principles go.