Can a bystander escalate a violent situation as a way to punish the victim or the person who needs help?

This is a oddly specific question. And this is also going to be a oddly specific post too.

Hypothetical story:

Let's say we have three characters here. Johnny, Sally, and Duncan. Duncan is the type of person who wants to be left alone, and doesn't like people. He hates being around people.

One day Sally goes to the gas station. And then Johnny opens the door for her. But Sally doesn't say thank you. This upsets Johnny to the point he says "you didn't say thank you ungrateful piece of shit". Then Sally starts to get scared, since Johnny starts to go in her personal space, and he is very angry. Johnny starts to touching Sally.

Duncan is also in the gas station store while this situation is happening. And everybody is looking at Duncan to help. But Duncan hates the fact that people are expecting him to help Sally.

So Duncan gets involved, and he ends up breaking Johnny arm. Everyone can see Johnny bones being out of place. It looks brutal. And then Duncan storm out the store with a very angry face.

Now back to the title.

Did Duncan does this on purpose? As a way to punish Sally for needing protection in the first place.