If someone does something you would not, that is not a good reason to go and do it yourself.
Sticking to your values, your best behavior, the best version of yourself is easy when everyone around is nice, pleasant, and accommodating. But can you do it also when you meet the asshole, when your boss sets a negative example, when your group is up to something you would not normally be proud of?
They started it! is a weak excuse for grown ups.