for an "evil act" to be defined in the game as an action undertaken with evil intent, the core books would need to include a definition of what it means for a character to be evil, which they do. But for an "evil act" to instead mean taking an action that is inherently classified as evil regardless of context or motivation, as you suggest, the core books would need to include a definition separate from these factors that explains what it means for an action to be evil, which they don't.
They do define an evil act. I quoted the d20 definition upthread: an evil act is one which debases or destroys the innocent.
Gygax defines evil in this way:
[T]he tenets of good are human rights . . . each creatur is entitled to lie, relative freedom, and the pursuit of happiness. Cruelty and suffering are undesirable. Evil, on the other hand, does not concern itself with rights or happiness; purpose is the determinant.
Hence an evil act is one which is rights-violating, or otherwise inflicts suffering when suffering was not warranted.
The two definitions are probably not strictly equivalent - for instance, the d20 definition seems to suggest that it is never evil to inflict suffering on a non-innocent party (so eg cruelty towards the wicked might be permissible in some cases), whereas Gygax suggests that even when suffering is not rights-violating it can still amount to evil. But they are near enough for practical purposes.
legal systems have long distinguished between what one knew/understood and what one should have known/understood. Only the former is considered "willful", but this hardly means the latter is "inadvertent," only less malevolent and hence less "bad" according to EGG's view of nobility as articulated in the traditional paladin code.
Are you a US lawyer? In which case you have identified something about US law that I didn't know.
I had always assumed that negligence in US law has the same meaning as in Anglo-Australian law - where it means something to which a person was inadvertent, but ought not to have been. Negligence is not a species of malevolence, in the sense of subjective malicious intent - it is an objective standard to which non-malevolent conduct is held.
In private law, knowledge is generally defined so as to include certain sorts of imputed knowledge (eg a party is deemed to know X if s/he knows facts that would alert an honest and reasonable person to X, or would put an honest and reasonable person on inquiry as to X.) However, knowledge in this imputed sense goes beyond wilfulness, and so this cannot be the sort of knowledge that is meant in the paladin context.
In criminal law, between knowledge/intention (which in English law tend to be the same thing, but are not always the same thing in Australian law, depending on jurisdiction) and negligence there is recklessness. This can be defined as "knowledge of a risk which ought not to be taken". So recklessness, which is a species of malevolence, is also a species of wilfullness.
A paladin who acts recklessly - ie who runs an unjustifiable risk of debasing or destroying the innocent - loses paladinhood. (Whether that counts as
motivation is itself a debatable question - in some systems knowledge of an outcome is equated with intending that outcome (so-called oblique intention) but in others (eg the criminal law of the state of Victoria in Australia) it is not.)
A paladin who inadvertently - ie not wilfully - brings about an evil result need not forfeit paladinhood. This shows that evil
cannot be solely about motivation. For instance, a paladin who kills an innocent person while honestly and reasonably believing that person to be a succubus or doppelganger in disguise has done an evil thing. But given that it was neither knowing nor wilfull, the paladin will not forfeit paladinhood. (Though I imagine many GMs might require Atonement, which has as one of its function absolution from evil deeds committed unknowingly or unwillingly - which would inclue evil deeds done out of good motivations.)
EDIT: This also shows that there can be "moral luck" in D&D. If the paladin honstely and reasonably, though mistakenly, believes that a person is succubus in disguise, and kills him/her, then the paladin may have done an evil thing (though without it being knowingly and wilfully done). But if, unbeknownst to the paladin, the victim of the mistaken killing is in fact a
doppelganger in disguise, then the paladin has not done an evil thing - but rather has done a different non-evil thing from the one s/he intended.
