Yes, marginalized people will not be discussing whether their marginalization was a good things. However, there are neonazi or Nazi sympathetic subcultures, some of them fairly large. It would by incorrect to then state that it is universally agreed upon that nazis were evil and a black mark on history. Similarly, there is a significant movement in the United States that either waxes nostalgic about or outright prefers the pre-civil war era with its slavery.

I understand what you're saying, but I don't think such people's hateful opinions should be counted when speaking about such things. There are always going to be people who do not think murder, slavery, and torture are not wrong but right. They are a small minority and while it's important you make note that such scum and their opinions exist, it's also important to note that they do not represent a significant portion of people either in number or in strength of opinion. There will always be outliers but I'd say most people and all decent people agree that slavery and the nazis were both wrong.