When does a person cross the line from just being mean, into somebody who is dangerous?

Based on the TedTalk with Dr. Philip Zimbardo, the person who did the Stanford Prison Study. What is the tipping point? When does a person cross the line from just being mean, into somebody who is dangerous?
1,000 words
Use the antagonist that you have worked up or develop another. https://youtu.be/OsFEV35tWsg > This is the Ted talk its referring to. You can develop a antagonist since I don’t have one. You’ll be writing about someone who basically crossed the line and turned into someone really dangerous. (No Cuss Words)

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