When someone brings a knife to a fist fight, then the temptation is to grab a gun. If you bring a gun though, your opponent will want one up you with a grenade. And so on.
Why is it that once someone is willing to cross the line, it’s easier for us to do the same?
That permission to do something out of character confirms what we have thought of this whole time. The difference now is that once someone has wronged us we feel justified to break the rules too.
Proliferation is a lousy tactic to solve one’s problems. Just because someone else is doing it doesn’t give us the right to add more friction.