Another factor that is almost as dangerous, is when some group (or a powerful individual) lose touch with reality. Lets say Putin really thought that most people in Ukraine want to "re-join" Russia, and that only a small group of "Nazis" prevent that.
Then there are individual episodes, like Unabomber, where someone on the fringe (maybe with some mental disorder, a psychopath or crazy fundamentalist) does something that seems evil. First of all, these are usually relatively minor in their effect, but can be blown up a lot. I'm more worried about our reactions to such events than the events themselves, in that if we are too quick to place the responsibility of such events on some group those people are associated with, the act of an individual can make us hate the whole group.
Finally, you keep coming back to eugenicists. There are certainly plenty of examples of "eugenicists" that did some things that we consider highly destructive today and others who did things we consider immoral today. Then there was some a large group of people that didn't actually do much, but that were worried about a dystopian future on the other end of a dysgenic development. Finally, there were plenty of scientists doing science in the field, ranging from pure pseudoscience to some of the most important discoveries of the 20th century. Some of these may have been hardline racists, others very liberal.
Finally, we must consider that eugenics, in some moderate form, had very wide support in many western countries for almost a century, even into the 1970s in many places. Even if we don't share those values anymore, labelling whole generations of people as Evil because of this is like labelling large parts of the population in countries where racism is still tolerated, as Evil.
And from a purely descriptive point of view, I would even say that the concern that caused the eugenics movement were not totally unfounded. At a time when most people lived on a farm, many people had a direct relation to the importance of which animals were allowed to reproduce and which ones were not, on the farm.
In particular, this was relevant for those who wanted to build a welfare state where "natural selection" on humans would be eliminated (this could be a socialist or social democratic welfare state, but also a national socialist welfare state, as seen in Germany). The kinds of pure capitalist societies that were common during the 19th century did come with some kind of social Darwinist natural selection built in, so was not seen as likely to have to add explicit eugenic policies to compensate for the welfare state.
As it turned out, it seems that a dysgenic drift in our DNA appears to be too small to be measurable. We've even had the Flynn effect, where it has appeared that mental ability has been going up generation after generation.
Let's imagine that we'd seen the opposite, where countries with welfare states had a radical drop in mental ability over a few generations (as the eugenicists predicted). Lets say that led to those countries losing their ability to organize as advanced, liberal democracies and advanced economies. At that point, eugenics might return.
So to sum it up, my view is as follows:
1) Try to avoid seeing people as Good and Evil. Know that if you do that, you are being illiberal, and you may make provoke needless conflict.
2) Understand your values and the values of other groups and individuals that you may have to deal with.
3) Figure out what differences you are willing to ignore (possibly only if they stay at some distance from you). Liberalism is precisely the willingness to ignore minor value differences. Illiberalism is to demand that the people around you have the same values as you.
4) Figure out what values you are willing to have a confrontation over, and whether you will only go into a confrontation defensively, as a response to an attack, and what values are so important that you need to actively try to impose them on others. Also, figure out the level of intensity you are willing to use in your confrontation, ranging form purely verbal, through imposition through the legal system all they way to the use of violent force. All these considerations need to include some cost/benefit considerations, the details of which are far beyond the scope here.
5) Know that even if you do end up in some confrontation with the opposing group, that doesn’t make them Evil, and it doesn’t make you Good. By giving up this excuse, you may discover something painful, namely that you are acting in ways that do not correspond according to a persona you have taken on as a pro-social, liberal and non-violent individual or group. You may end up doing things that you never thought you would do and may face cognitive dissonance and perhaps an existential crisis (often a cause of PTSD).
Now, without resorting to the Good vs Evil narrative, you should figure out what it is about your values that cause you to be willing to fight the conflict. If you cannot justify your actions from your values, you are being irrational, and should consider resolving the conflict.