it's reality

Just declaring it to be 'reality' doesn't stand in for an argument. I obviously disagree so if you want to have a discussion you have to forward some rational argument for your view.

Why will withholding a vote when neither candidate is acceptable not stop genocide?

You've simply declared that it will, but not given any reasons.

If both candidates are going to continue arms sales, then there's no difference. The idea that Trump's going to sell more is silly, there's no current limit, Israel buys what they need. So the only affect I can have is in the long term.

Here, there's two options:

Make it clear that genocide does not win votes.

Make it clear that even genocide is not going to dissuade me from voting Democrat and so give them basically a free ticket to ignore voters complety.

The former is the most likely to stop genocide.

Same goes for any other issue.

All the while you vote as if it were a duopoly, it will remain a duopoly. It's not about getting 'the least worst person' into power next month, it's about the long term value of making it clear to politicians that they cannot simply threaten us into voting for them, that they need to present policies we want in order to secure votes. Anything less and you might as well chuck democracy now. All they have to do is build up the bogeyman again and you'll vote for them no matter what. In what way is that remotely "for the people, by the people"?