Some people will take action when they witness domestic violence, even if it involves complete strangers. For instance, things turned unruly at a car meet when a man slapped his girlfriend, sending the gang of witnesses into a rage that culminated in the flipping of the asshole's car. He had already fled, making it even easier for them to literally turn his world upside down. However, everyone peeled out as soon as they heard sirens.
Good luck explaining this one to your insurance provider.