Rav Avigdor Miller was asked this same question:
Q: What is the problem with looking at the face of a wicked person? [אסור
לאדם להסתכל בצלם דמות אדם רשע – “It is forbidden for one to gaze at
the face (image) of a wicked person” (Megillah 28a). ]
A: If you look at the face of a רשע, you become a רשע. And I’ll tell
you why. We are influenced by ...