There are some instances where this is justified (e.g. Nazi Germany). If you look back in history, there are many instances of evil committed by most nations.

However, one has to be careful with conflating hyperbole with facts. A lot of very heinous crimes aren't committed outright; there is always some veneer of righteousness by demonizing the other. If you believe that "pure evil" exists, then you're more likely to believe such a narrative and maybe take part in the action it calls for.