Ultimately i think that the only way you can decide upon whether or not a creature is evil is through your own cultural context and not through a defined universal sense of what is or isn't inherently evil. Mindflayers for example are raised in an isolated collective where they all share thoughts, experiences and opinions more powerfully with each other that they can with any other sentient race. Understandably they percieve themselves as superior to other races, and as such probably view the devouring of other sentient being's brains as somethin akin to the butchering of animals for sustenance. In their view they would see themselves as good and view people who oppose that viewpoint and deny them access to that sustenance as denying them a fundamental right.
There are numerous examples in Forgotten Realms lore where mindflayers have afforded a healthy respect to beings of other races, particulary psionists, who they can relate to in a manner similar to their own race.
True Evilness can only arrive when the mature Mind Flayer thinks back to how it destroyed a sentient intelligence to acquire a body, and how it's been feeding off the meat of intelligent beings its entire life, and then it evaluates that as being a perfectly viable and acceptable choice of lifestyle. That's Evil.
That would be just like a human that had a stint as a vego...
They are clearly deserving of noble wrath.
As such mindflayers are not inherently evil, but merely of a differing opinion, as a product of their upbringing and social connections.
Were beings to be inherently evil however, would it not be best to consider CHARNAME? I doubt that their can be anything much
more evil than the God of Murder, whose purpose of existence is solely to condone assassination and immoral killing. If anything could be considered inherently evil than it should be CHARNAME him/her-self as they are born into the domain of the go dof murder. Creatures like Sahaugin have no irrevocably evil ancestor, that is solely served by destruction of everything else, whether they are in opposition or not, or shameless self advancement beyond thought for others. The Sahaugin show that concern at least for their race, and the survival of loyal members of their species. This differs greatly from the God of Murder. Yet CHARNAME can become a paladin and get a reputation of 20 and hot virtue scores...
So therefore a creature cannot be judged as inherently evil, however the CHARNAME's actions in killing the various creatures of the Forgotten Realms, and in completing quests can be judged according the precepts of the larger populous of the surface communities of Amn (in particular humans, and humanoids). Their view that each particular monster is/isn't worthy od death is what has been attempted to be implemented in the Infinity Engine...
(Breathe)
-oswald
EDIT>>>>>SORRY....i think i repeated some stuff