Loghain the slave trader is apparently a grey character because he had a really justifiable reason for leaving Cailain to die before starting a civil by demanding that the Landsmeet bend the knee to him, imprisoning his own daughter, poisoning Arl Eamon, and teaming up with Howe *after* he learns Howe is a traitor (since apparently they didn't team up).
The only reason he counts as grey is the lack of moustache twirling.
I agree to a large degree, but the slave trading always remained with me because with all the other things I could kind of see how it might fit in to Loghain's more and more desperate actions and removing those he saw threaten Ferelden, even if it was becoming really ridiculous at a certain point. Yet with the slave trade, it just kind of happened, didn't fit at all with the other actions and was there to just underline that this was a villain that needed to be taken down.
For me, with Meredith, Arishok, and as the lesser villain Petrice, I could understand all of their actions from the perspective of the characters and while some of their actions were truly horrific, it fitted with their world view, it was justified within. I could see how they were heroes of their own stories. With Loghain, to me he just felt like a power-hungry madman with really no personal justification to several actions and thus he never really felt grey for me.
Now, I'm not saying everyone should feel similarly, as characters and how they appeal to people are so subjective and dependent on so many factors, yet most of the time I see Loghain discussed there is so much praise for him with so many of his deeds basically ignored, yet Meredith, who I freely admit was not developed as much as she should have due to the rushing of the third act, is just slammed as a one-dimensional mad woman.