I think calling the police is the right thing to do.

I also think exercising caution is the right thing to do and not getting involved.

Remaining discreet and watching allows you to offer essential testimony that could convict someone.

I have witnessed violence and pretended to ignore it, but then concealed myself in a shop doorway and called police and kept an eye on things, as a result of which the criminal went to jail.

The police need a case.

For that they need evidence.

An impartial eyewitness is a great source of evidence.

A dead vigilante is a bad witness ... and a lost family member ...