Oh look, matriarchies can be violent (just like any other human social setup)!
I admit, I write a lot of violent matriarchies. Mainly because I believe that any social setup that represses another group within it is going to be violent. The most egalitarian fantasy group I have are a bunch of polyamorous, socialist, gender-fucking hippies called Drakes (in, fittingly, The Dragon’s War), and they’ve managed to keep their society that way mainly based on environmental isolation.
But, my matriarchies?
Violent as all get-out.