I think it's logical.
The Shadow curse came from Shar. The Shadow inn with Selune priest Isobel resisted it somewhat. I think it is totally within logic that when you kill the Selune Asimar, lover of Isobel, Shar's curse breaks the barriers set up by Isobel and returns full force.
Making Shadowheart Dark Justiciar, is equal to, taking the side of Shar.