As for why; it's because evil choices are designed as punishments for the player to roll with for straying from the heroical path, rather than branching the story out in interesting ways and introducing the player to unique paths filled with content one normally would not have access to. That's why majority of evil choices do not have any satisfying outcomes because they're just consequences that lead the player down the exact same paths as heroical choices (naturally with differences reflecting both choices), but drastically less content to engage-in afterwards.
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.
For me that explanation is more of trying to add theory post-factum - you already have an outcome and you need to find justification for that because of time constrains or whatever prevented something more elaborate. The game is showing you that there are no black and white situations (or at least trying to ;)) and that approach should work on both sides of the spectrum. Minthara is a great example of that.
There are quite a lot of things that are clearly left unfinished or simplified. For me the outcome and the fact that there is no clear explanation suggest that's the case here as well.
Isobel was channelling from her goddess. Not from Nightsong. I'd rather go with the explanation where because of her death, Shar got more power over shadow curse, which was enough to break the barrier. But this also is trying to stick theory to the outcome instead other way around - actually at the moment of Dame Aylin's death, there was no mention that she is tied to the Selune in any way. But I may be wrong as I don't remember each line of dialogue, especially since I had different on each run.
I was looking on the web about this outcome to check if I missed something and found people being surprised as well, which suggest that the whole thing is not properly designed, not to mention - sad.
