TV show or possibly film. Bad guy is killed by woman with gatling gun whilst hiding out at militia he created.

Angelina Jolie uses a minigun in Mr. & Mrs. Smith, but they're not attacking - they're being attacked.

No, not "Mr and Mrs Smith"
A little more I think I remember. Names are made up for narrative clarity, they are not the actual names.

At the beginning of the series/film Anna and Bob are part of some organisation, I think there are about 7 members. Charlie is not a member of the organisation but somehow ends up butting heads with them. Charlie and Anna are definitely good guys and I think the organisation is ostensibly good. Over the course of the series/film the other members of the organization die. Bob, who has been creating a militia as a side project ends up being revealed as a bad guy and rather than take on Anna/Charlie goes to hide out with his militia.

In the scene I'm remembering, the militia guys are all formed up in a courtyard when from another angle we see Anna come out of hiding with a massive fast firing gun, taking them all by surprise and downing them all before they can recover. How she actually infiltrated is left unseen. Bob is then killed, (by Anna or Charlie, can't remember which)

Could it be "Bad Girls" (1994)?