Molly uses Avada Kedavra to kill the chickens because she hates them. She hates what they represent, that good and upright men like Arthur make no money but the Malfoy's have all they need.
She's got so good at it now she can do it silently, so the kids don't know how much she hates being poor.
It's a skill that comes in handy. Bellatrix doesn't know what spell is coming because Molly never speaks the words. All that Bellatrix sees is the green light.
Even easier than killing chickens thinks Molly. At least I don't have to pluck her.
