Far Realms inhabitants have no essence from the D&D multiverse's alignment forces. They are not native to the Great Wheel. Their reality is not governed by the same forces, otherwise Material Plane natives would be able to comprehend them and not be driven mad by their mere presence/gaze/telepathic contact.
The multiverse has these alignment forces as actual forces, not just perceptions or repercussions; a Demon is Evil because it is literally composed of Evil to some extent; a mortal becomes evil by allowing that same essence amidst the multiverse to influence his/her/its actions, and thereby absorbing some raw Evil into themselves from the environment (just as a wizard might cast an Evil spell, such as Animate Dead, using the magical energies of the surrounding area; which must obviously include some Evil energy).
At least, it's something like that. Good, Evil, Law, and Chaos are incarnate forces that make up large, even infinite, portions of the D&D multiverse. They are not mere matters of perception; Bob may hate his neighbor Joe and think him an evil SOB, but that doesn't mean Bob's casting of Holy Word is going to smite Joe; because Bob's perceptions and beliefs, no matter how strong, do not make Joe evil, not even to Bob's own magic spells. Indeed, Bob himself may be evil (in which case he couldn't cast Holy Word, but still...), and Joe could be good, despite Bob's skewed beliefs that Joe is evil and Bob is good.
A Far Realms inhabitant does not carry any essence of 'evil' or 'chaos' in it, because evil and chaos either don't exist in its reality, or have different definitions there, and do not synch up with the multiverse's forces of evil and chaos. The creature is incomprehensible and anathema to normal reality in the D&D multiverse. It holds no essence of Evil from the multiverse; it's own essence may be anathema to the intrusion of such Evil essence.
Even if it was the Far Realms' equivalent of chaotic evil, the multiverse wouldn't be able to identify that, let alone manage to force some of its own Evil energy into the Far Realmsian horror and expect that energy to stick around; the critter's own essence may be like antimatter to it, destroying or expunging the 'alien' energies of the multiverse from its body. Without recognizeable Evil taint in the creature, a cleric's Holy Word or Detect Evil spell won't be able to identify the target (and react violently to any Evil essence in it; since it has ^!&snarf7*(4)a1!>? essence instead of Evil essence, there's no discernable Evil within it for a Holy Word to smite).
A Far Realmser may act in what mortals define as an 'evil' manner, but it sure as heck doesn't understand the Great Wheel multiverse's concept of 'evil'. It would be like a colorblind dog trying to understand color, as you point out objects and describe their color; he sure as heck doesn't see any difference. And he probably doesn't know what you're saying; you're speaking in wierd barks, yips, and whimpers as far as he's concerned.
The Far Realmser, basically, is no more susceptible to alignment than a common animal or insect. He/she/it/blue/12/!%!^ isn't going to comprehend morality any better than a simple-minded creature would; not because he's stupid, but because his concepts of reality are wildly different and alien, with little or no similarities.
As far as he knows, you're no different from the 'dirt' or 'grass' equivalents in the Far Realm; he may believe the raindrops falling from the sky are more intelligent than you, because to his idea of reality, perhaps everything is alive, and there's nothing special about being alive; ergo nothing has any intrinsic value, and you're no more important or sentient than a rock or a spider or yesterday's lunch or the turd that was yesterday's breakfast. :\
To the Far Realmsian concepts of reality, mere sentience may be without value, and only gargantuan, *nth-dimensional thinking capacity qualifies as 'sentient' and possessing value. And in their reality, that means anything similar to humans in thought capacities is, compared to everything else in their reality, as offensively stupid and pathetic in mental faculty as a dung beetle. Their freakin' air particles are probably sentient and constantly screaming in protest to the gargantuan monstrosities that inhale and consume them every second.....
