One thing I love about this interaction is that we have Tom trying to make friends with these two younger gargoyles. The youth of these people are so ready and willing to make good with each other.
But here we have this protective mother running to her son, immediately assuming he's in danger. Brooklyn tries to reassure her that there was no design on harming the boy; her fears were unnecessary, and he approaches in as non-threatening a manner as he can manage. But the woman throws and strikes him with that wooden stick she's holding.
Brooklyn is young, and he is hurt. He looks so wounded in that moment, but he doesn't yet look angry. His emotion has settled yet, and an adult of the gargoyles groups swoops down and puts herself between the humans and her clan. She is angry.
We already know that Brooklyn is of the point where he's ready to feel like an adult in the clan, and this is how the second in command acts. She responds with anger. So he does too. Lexington, not to be left out, joins in.
They believe them monsters? Fine, they'll be monsters. I think it's so interesting that Demona asserts that no, the humans are monsters. She actually believes this. I think it says something about Brooklyn and Lexington that they decide to 'go along' with the monster thing. They don't believe any of them are monsters; they're just hurt and angry.