For two years now this weed has grown against a large rock in my lawn. Last year as I was about to reflexively pull it, I was struck by the brilliant contrast between the green foliage and the vivid violet stems and gave it a pass for a while. Later in the year it had white flowers followed by deep purple fruit. I have no idea what this is and the other one or two examples I've seen don't have the amazing colors.

Does anyone know what this plant is?

Thank you