I don't think a lot of people have read the direct literature about the peers of Charlemagne.
Sure. But I think LotR (with knights like Aragorn and Eomer) and some version of King Arthur are fairly well known by FRPGers (and at least as well known in the late 70s, when paladins were invented as a class, as they are today).
There is a lot of room to vary on what exactly constitutes an evil act.
From the 1e Paladin class description "If they ever knowingly perform an act which is chaotic in nature, they must seek a high level (7th or above) cleric of lawful good alignment, confess their sin, and do penance as prescribed by the cleric. If a paladin should ever knowingly and willingly perform an evil act, he or she loses the status of paladinhood immediately and irrevocably."
I think that "chaotic" act is problematic in AD&D because it's not terribly well defined. So I'm not going to try and defend the coherence of that - I think it's close to unworkable. But evil is given a pretty clear meaning, by Gygax in his DMG (p 23):
the tenets of good are human rights, or in the case of ADBD, creature rights. Each creature is entitled to life, relative freedom, and the prospect of happiness. Cruelty and suffering are undesirable. Evil, on the other hand, does not concern itself with rights or happiness; purpose is the determinant.
So whenever a paladin acts in disregard of the rights or wellbeing of others, that is an evil action. Further text on good and evil alignments in the PHB and DMG indicate that scorning truth and beauty are also evil acts.
It's possible to concoct scenarios - well-known to philosophy students! - where no-win pressure can be placed. For instance, is it good or evil to switch the careening trolley onto the track where it will kill one person, rather than leave it on its current path where it will kill five people? Switching is clearly sacrificing the wellbeing of the one; but there's a tenable argument that not intervening is a cruel indifference to the wellbeing of the five.
But these sorts of situations are, I think, not that likely to come up in FRPGing unless a GM goes out of his/her way to make life hard for the paladin: and if the paladin player then makes a call - to switch (
the needs of the many outweigh that of the one) or to not (
it is fate, not me, who killed you five!) then it seems to me that there is no need for the GM to second-guess or superimpose his/her own view.
The very existence of a LG knight class tells us that defensive violence is not impermissible. Consensual violence - duelling etc - is probably suspect (it lends itself to showing off, and doesn't contribute that much to happiness) but a villain who offers to free the hostages if defeated by the paladin in a duel seems like fair game!
Punitive violence is more tricky - one of the paladin level titles in AD&D is Justiciar, and in the film Excalibur when Arthur is knighted the ritual confers "the right to bear arms and the power to mete justice"; but I can imagine many players who would find retributive violence distasteful at least. Again, this seems a place where the GM can follow the player's lead. There is nothing in the concept of good vs evil action, nor inherent in the gameplay conceits of D&D, that dictate a particular answer here.
From the 3.5 PH paladin description "Paladins must be lawful good, and they lose their divine powers if they deviate from that alignment. Additionally, paladins swear to follow a code of conduct that is in line with lawfulness and goodness." and "A paladin who ceases to be lawful good, who willfully commits an evil act, or who grossly violates the code of conduct loses all paladin spells and abilities (including the service of the paladin’s mount, but not weapon, armor, and shield proficiencies)."
3.5 "“Evil” implies hurting, oppressing, and killing others. Some evil creatures simply have no compassion for others and kill without qualms if doing so is convenient." Paladins generally do a lot of hurting and killing as divinely powered knight champions, sometimes without qualms or compassion for evil creatures when fighting evil.
This is another occasion where I think the 3E presentation of alignment is markedly weaker than Gygax's. Gygax gets to the heart of it when he says that, for evil,
purpose is the determinant ie the evil person does not acknowledge other people, their rights and interests, and other values like truth and beauty, as limits on their will and desire.
The paladin who commits an evil act, then, falls into pretty clear tropes: wrathful killing (ie out of hate or anger rather than defensively or in just retribution); wanton or prideful infliction of violence or withholding of aid; giving advice to others not out of sincere belief in its helpfulness but because of the benefit that the resulting act will grant to the paladin; etc.
In real life it's easy to fall into at least some of this sort of conduct because we're fallible humans ruled in part by our passions. But in the context of typical FRPG gameplay I think it should mostly be possible to avoid such conduct.