I wouldn't call either Mephala or Boethiah "good" just because they are revered by the Morag Tong/alot of Dunmer respectively. Morag Tong is a clan of Assassins; they act as an "above the law" establishment in Dunmer society to get rid of people they don't like, pretty much. A very dark shade of grey if you ask me. And both of these princes are lords of deciet. Though deciet might not necessarily accomplish something one would consider "evil", these guys revel in it. They love it when a man turns on his brother out of greed, as an example. Is taking amusemant in this not evil? Furthermore, history has made the Dunmer people rather bitter and spiteful, so I wouldn't really look to them for moral authority.
So, end-quote-reply. Furthermore, I must say that I disagree with constantly trying to discredit the usage of the word "evil". It definately applies. Black and white are still part of a grey scale. You can certainly argue that Vaermina, the lord of nightmares, is not evil. But from my point of view, nightmares and the untold horrors that Vaermina represents, is indeed an "evil" concept. Then there are those that you couldn't possibly argue against beeing evil. Of course, I am speaking of Molag Bal. He is as black as black gets, there is no doubt.
So, my point is: why can't we just name evil for what it is and accept that? There really is no need to vindicate anything, it's just fiction.
Because evil and good are subjective. Malog Bal is the Daedric lord of domination and thus can easily fit into animals when they fight for dominance and Mehrunes Dagon is the Daedric prince of destruction which sort of fits into the philosophy that new life can rise from the ashes of destruction.
Hircine = Daedric prince of the hunt = huntsmen get his blessings
Nocturnal = Daedric princess of luck so you do the math
Azura = Daedric princess of dusk and dawn but still somewhat occupies a gray area by Daedra standards
Boethiah = Daedric princess of deciet and sometimes that is a good thing (would you give top secret information to your enemies and allow them to destroy your nation or would you save your nation by lying to your enemies?)
Clavicus Vile grants wishes but there is always a catch as in he does a favor for you and you have to do a favor for him
Hermaeus Mora = Daedric prince of fate (hint hint)
Jyggalag = Daedric prince of order (hey guess what even order can be a bad thing)
Malacath (whose statue in Skyrim looks a bit like Darth Maul for some reason) is the patron of the spurned and ostracized
Mephala is just plain confusing
Meridia has a strong dislike for the undead
Namria is the Daedric princess of Decay/Death and that is in nature too
Peryite is the Daedric prince of tasks
Sanguine is one of those darker shades of gray daedric lords
Sheogorath = Daedric prince of madness which isn't always a bad thing
Vaermina is again one of the daedric lords that is a darker shade of gray
The point is that good and evil for mortals is not the same as good and evil for daedra.