I've got the bad one and here are my thoughts on why the three endings seem illogical to me.
First off, the DLC is called Blood and Wine and CDPR sure made it so everyone notices this. Everything and everyone is connected with blood... well, and with wine
Syanna and Anna Henrietta, Regis and Detlaff, Geralt and Regis... And also this new "rule" that came completely out of the blue: only a vampire can kill another vampire. It's emphasized on so often and so strongly, I expected it to be the major player choice of the DLC. But instead... instead this main truly painful moment is fully decided by developers and not by the player.
When I came to Unseen/Syanna choice in my playthough, my first thought was "this is it, this is the deciding moment". I knew Syanna was behind those murders, I kinda guessed she planned to go for Anna (political pamphlets all over Toussant, Syanna being Anna's "long lost sister", her desire to have things only her sister has access to...), I knew she used Detlaff and I knew Detlaff was more of a beast than a human, so it was pretty obvious he's going to kill Syanna for what she did to him. At this point I decided Detlaff was too dangerous to let him go and this is were I choose consciously (at least a though so) to condemn Regis, because it was hinted several times that only Regis would be able to kill Detlaff and that he'll be condemned by his kind for doing so. And although Syanna was yet another evil person to me, I thought it was inhuman to just set her up for being murdered by a vampire. Geralt is a witcher after all, he has to protect humans from monsters. Syanna deserved to serve time in jail, yes, probably even death, but not this kind of death.
So my Geralt made a conscious decision to risk his life to protect Syanna and to condemn his dear friend to kill the monster. After killing the beast he found the missing letter and it was indeed about Anna's murder. After talking to Syanna I was pretty sure she will try to do something as she was obviously the type that never gives up (she says it clearly). So my Geralt warned both Damien and Anna about Syanna. Yes, Anna feels guilty for what happened to her sister, so she refuses to accept the truth, but Damien and Geralt himself should have stopped this! Geralt could have stopped this as witchers are inhumanly fast! But no: no matter what you do, you can't prevent bad things from happening. There's simply no choice for the player.
Of course, after getting the bad ending I decided to go for a good one. Only to find out that bad ending is indeed a bad (short and incomplete) version of a a good one. And only to see that I have no option for conscious decision about Regis: he kills Detlaff anyway, no matter what I think or want. And imagine my frustration that after finding a third ending I realized that such an important matter as Regis' fate is decided randomly by some ribbon. They made a big deal about this and then it all came down to that ribbon. Wow. Just wow. For me this, and not even the fact that the best quest of the expansion locks you into the worst ending, was the biggest disappointment.
Also, I think that in the main game the bad ending is the one that has the most emotional impact on the player. It's bad, but it's brilliant. And in DLC the bad ending doesn't have a value of its own. What it does is it just makes you reload and pick a different dialogue option.