It doesn't matter how nice and calm you are, everyone has their breaking point. Since people with reputations for being kind and mild-mannered are often taken advantage of by those who underestimate them, it can be cathartic to witness them fight back.
In a popular Reddit thread, people share stories of seeing a nice person "snap." In most of these cases, their behavior was completely provoked.
I had just started high school and there was a new kid (I was from a one primary/one high school town, so all the kids already knew each other). He was a giant aboriginal kid who was quiet with no friends, so my friend and I befriended him. We always chatted to him and he was so lovely, always softly spoken and generally just so nice.
Like holding doors open for us and bashfully smiling when we would go out of our way to be friendly to him. Anyway, he started getting picked on by the “bad boy” of our grade relentlessly. One day he snapped and beat him up with a garbage can and was expelled and I never saw him again.