You aren't asking a big enough question to actually grasp what the lesser evil even is.

The first question you really want to ask is "what is the primary contradiction?" We can discuss what that might be, US hegemony or Western imperialism or neocolonialism, but this will inform our understanding of how we should vote. Only once we identify the primary contradiction should we cast our votes in whatever way will advance the struggle against it.

Let's say we identify US hegemony as the primary contradiction. Does voting for Harris help advance the struggle against US hegemony? No! She would have been a fine steward of the empire, if she was president the hegemon would be in better shape than it is under Trump and the struggle against it would be even harder.

Trump is wrecking the empire, and in light of this, Trump winning is actually a lesser evil. What we see under Trump is the US pulling back from all of its soft-power while threatening its own allies. Yes, it has become more violent, but the violence isn't a sign of strength. The US is in a weaker position than it has ever been in my life. All is chaos under heaven, the situation is excellent.

But, if Trump winning was the lesser evil, does that mean we vote for Trump? Also no! It's important for him to not actually ever be popular within the US, because that also erodes his own legitimacy among the US population and makes the US itself harder to govern. The fact that he "won" with 49.9% of the vote isn't the best outcome, better would be losing the popular vote again, but it's still good because forcing USAmericans to confront the weakness of their elections also weakens its position as the hegemon. They look at their own elections, look at Trump winning twice without ever getting a majority, and ask "do we live in a democracy?"

(The answer is no by the way.)

His unpopularity is why there was an uprising against ICE in Minnesota. There wouldn't even be protests against ICE if Harris was president, and there weren't under Biden despite him deporting more than Trump in his first term, but under Trump people can easily see what they have always been: colonial occupation troops. Under Democrats the streets are empty, under Trump the people fight back. The BLM uprisings happened under Trump, but it was Bill Clinton that gave us the Violent Crime Control and Law Enforcement Act of 1994 to fill the prisons with Black people. This has to be factored in when we ask what the lesser evil is.

They're both great evils, so identifying the primary contradiction is the only way we can identify what is to be done.

Notably, this is not accelerationism. The goal here isn't to make things worse so they get better. The goal is to make the US empire weaker, because we have identified US hegemony as the primary contradiction and that's what we are struggling against. Revolutionary defeatism, in other words.

The Democrats are already guilty of genocide, both parties are the greater evil. Why is it not even an option to try and push the Democrats slightly to the left of fascism?

The lesser evil is actually a mass armed uprising, but you privileged fascists-in-denial reject that course out of hand because it might personally inconvenience you, and would rather knowingly support Holocaust 2 than lift a finger for humanity.