There are occasions where something bad happens and the evil is palpable; it imposes itself on my psyche just as much as the physical world does when I feel pain. As such, I can't deny the existence of evil anymore than I can deny the existence of the physical world. I mean this not just as a rational claim, but that I'm literally incapable of disbelieving it. I may insulate myself in privacy and theoretically question whether evil or the physical world really exist, but it never gets beyond that theoretical level. For example, on 9/11 the evil was tangible.
But an event doesn't have to be horrific and huge in scope in order for me to recognize it as evil. Sometimes it can be something simple and otherwise inconsequential. Maybe just the utterance of two short words. Like those the student utters towards the end of this video. It gives me chills.