I don't disagree with your idea of letting bad people kill each other off, but I don't think this is a battle between two evils. White supremacy, particularly in Portland Oregon isn't a threat. According to the ADL, there are less than 30k members of white supremacy groups nationwide, less than 3k in the KKK. the idea that this is a threat in Portland of all places is absurd.
What's happening in Portland is a group of violent fascists ironically calling themselves antifascists are violently oppressing anyone who doesn't buy into their worldview. They're a bunch of anti-government ideological idiots and spoiled children destroying the lives of everyday citizens and making Portland an unlivable city. And it's all based on the myth of institutionalized white supremacy. If it was a battle of evil vs evil, I would agree - have at it, they could all kill each other off and let God sort them out. sadly, the victims aren't the bad guys or at least not all of them, not even the majority.
In places like California where you have millions of idiots electing people to enact policies based on these same myths, I say go ahead, let them have what they asked for. Let it all turn to complete shit until everyone finally opens their eyes and realizes what they asked for sucks and in order to fix it they have to completely tear it down and start over.