Beings that are "always evil" implicitly cannot be beings that have the ability to choose. This can cash out in two distinct ways:
1: The naughty word, highly racist-like way, where the exact same phrases used to "justify" horrific acts against real people, past and present, appear and see unquestioned use. This is what D&D has done for most of its existence.
2: Actually using beings that have a choice, but choosing to live differently would clearly change them. E.g., illithids are what they are because they have chosen to eat sapient brains; if they were to eat animal brains or that brain-moss stuff, the effect on their bodies would become obvious with time.
As an example of my own, I recently articulated why devils and demons exist in my home game, Jewel of the Desert. This is a biased account, filtered through the lens of a specific dominant religion, but it is the first time the party has heard the story. Long story short, the One (infinite creator of all things) created many servants to join in making and guiding creation, but those servants were not permitted to force mortals to do good or serve the grand plan against their will. Eventually, a portion of the One's servants got fed up with never being able to enforce the plan, so they started doing so. There was War in heaven. Because of the nature of celestial beings, this war was infinitely long, but over in an instant from our limited mortal perspective. Those servants who rebelled became devils, and each and every devil you meet is the veteran of an infinitely long, horribly violent conflict specifically with the goal of being able to enforce their rules on others; it's not that they CANNOT choose otherwise, it's that they WILL not, because they're immortal beings who have staked a literal eternity on this particular position and if they could have been persuaded to choose differently they WOULD have during the war. Demons are those servants (notably from both sides) who came to enjoy the destruction and horror for its own sake, and are thus in some sense "twice fallen," since the devils at least still care about a plan, just not the One's plan. Devils were "cursed" to be exactly what they want the world to be: ruled by iron law and hierarchy, likewise demons are filled with desires that can never be sated because that's what they fought for. Again, these are beings who had an infinitely long time to change their minds and didn't--it is fantastically unlikely that they will change their minds in a finite span exposed to mortals.
(Though it is worth noting that the party bard is literally descended from a now-former succubus. She has come to love the mortal world for what it is, its fragile beauty. She only just realized that her long-dead husband is the one who gave her a new True Name, making her something....no longer demon, but not the celestial servant she was, either. Having passed her powers to said bard so he can use them for good, it is now possible for her to die a mortal death, which normally isn't something celestial-origin beings are capable of.)
The point being: these devils and demons aren't beings who "can't choose" in the sense of some inborn taint or whatever. They chose, and indeed chose so hard that it is very very close to impossible (certainly incredibly improbable) that they will ever choose differently--and making such a different choice would explicitly mean becoming something other than "a devil/demon." Sort of like the difference between a pickpocket and a magician: both may use the exact same techniques, but the former is a thief and the latter is an entertainer. To choose not to steal means to choose not to be a thief, and thus to be something other than a pickpocket proper.
Instead of "they have no choice, they are simply evil because they are," I wish D&D would consider "they have a choice, but choosing differently would make them something other than what they are."
