Anyway, show me a character who refuses to kill ANYTHING, and I'll show you a Good character.
I'm inclined to agree most of the time, though I'm not sure I agree that this is a requirement of a Good character and I'm not sure all pure pacifists are actually Good. This would come down to, "Is Pacifism a good ideology?" Obviously Pacifism is generally inspired by Good under the definition of Evil I've offered. If Evil is destroying things, then clearly Good is at least refraining from destroying things (which you'll probably note leaves a bit of room for Neutral pacifists). The question usually comes down to, "Do you do more good by refraining from destroying destroyers, or by destroying them to protect the innocent." This is a huge point of contention, and I can see it seriously dividing Good thinking types even nominally of the same alignment. Of course, if they are really Good, it wouldn't divide them to the extent of creating strife, hate or violence - but it would be something they'd really suffer over, and actively chastise and try to persuade their good allies over.
But in D&D, there are several much bigger exceptions possible than in the real world because there are just more different obvious categories of being. Pacifism usually is a promise not to kill people, and not merely to not to kill generally. It doesn't normally extend to everything alive. Sure, many Pacifists are also Vegetarians to avoid killing animals because they see that as an evil as well, but relatively few of those oppose say Antibiotics as if doing violence to a disease bacteria was evil. It's possible to be both a Pacifist and to kill.
In D&D, several of the foes are explicitly 'Not Alive'. Two obvious ones are non-sentient constructs and undead. You could smash a construct because it is merely an animated object, or you could destroy an undead, and neither would be technically killing anything. In the case of undead, you are possibly actually freeing something to live which is currently unliving. In the case of constructs, it never was alive. There are a couple of related fantasy situations. For example, it's typical of fantasy curses that the victim is trapped in a form they can't escape from unless it is slain - a cursed demon bear, a magical fox, a feral half-man, etc. They often beg the hero to kill them. In D&D this rather makes sense. Killing someone permanently is relatively hard. If you assassinate someone, well, then their friends can just hire a cleric to Raise Dead and your vengeance is made mute. But if you trap the person in the form of an immortal ooze, well, now that is perhaps easier and a truly terrible vengeance indeed. No afterlife for you. Eternal humiliation and suffering. So in such a situation, a person with a Vow of Pacifism (or a taboo) probably couldn't slay the magical fox just because it begged them to, but an intellectual pacifist that had an inkling of what might be going on would probably not think they'd violated their morals by slaying the cursed creature's present physical form. And making the condition for breaking the enchantment being that they could only be killed by someone that loved them, now that would to an evil being seem like a tough condition to break.
There are other examples of things you could kill that wouldn't violate pacifism intellectually. The biggest and most universally agreed upon category would be I think fiends. Fiends are incarnated destructiveness. I can't imagine many Good pacifists claiming that they had a right to exist or claiming that Incarnated Death was actually alive in the same sense a person was. (But see Neutral pacifism mentioned earlier). In my game, basically no one, not even a person who was a pacifist and abhorred killing would normally have a problem with destroying something like a Lesser Darkness Spirit, a Sin Spirit, a Horror Spirit, a Disease Spirit, a Lesser Curse Spirit, a Cannibal Spirit, a Greater Doppelganger or any of the other hundreds of common incarnated evils, nor would they generally have a problem slaying the servitors of an evil god (something like a Pit Fiend), which they'd see as a cross between an evil spirit and a really fancy construct.
Depending on your cosmology, Aberrations might fall in a similar category. In my campaign, Aberrations are things that weren't meant to exist that were brought into being through evil magic.
If you total this up, even a fairly strict good Pacifist could still kill Undead, Constructs, Aberrations and Evil Outsiders. That's 4 very common foes. If you throw into that that many Pacifists only care if you kill things that are people, and are not the sort that see violence to an animal as being the same thing (most Amish aren't Vegans), then you can maybe add to that list unintelligent animals, oozes, plants, and the less intelligent sorts of dragons, beasts and magical beasts.
In some campaigns, we've now listed the majority of things you'd actually have to fight. The real sticking point would just be, "If the BBEG is human, can we kill the BBEG and his minions?" And my feelings as a DM is that if you want to play a Neutral Good character, that this should be something your character really wrestles with and worries about, even if the character isn't explicitly a Pacifist. I'd love for a PC to come up with a character concept of a character that preferred non-violent approaches and generally tried to avoid doing violence to 'people' and worried about how 'people' was defined.
I should say that in my game, all Fey, Goblinkind, Elves, Humans, Dwarves, Orine, and Idreth (two homebrew races) are explicitly defined as people and almost universally accepted as people. A few fanatics deny that Goblins are people any more, and some deny that Fey should be counted, but most of them that argue that way aren't 'Good'. (A PC who was Good but believed Goblins weren't people would be an interesting concept in my game.) The biggest moral controversy is over 'Half-People' or 'former People' generally. What about lycanthropes? Pure monsters or should you always try to cure them? Half-fey are obviously people, so almost no one is thinking the child of a Selkie and a Human is a monster and not a person, but what about someone whose parent is a dragon, a fiend, or a genii? Are they people as well, or does even a drop of the blood of monsters make you a non-person? Are the genii basically Fey, and therefore people, or are they something else? Are sorcerer's people? Assuming a sorcerer can be a person, is there some line after with a sorcerer stops being people and becomes a monster? Does even a drop of people blood make you are person? All the giants have people blood in them, so are they people as well? What about the Gods? Are they all people, or should they be counted as just powerful spirits? That is assuming you could destroy an evil god, would it be murder or would it be no different than destroying a curse spirit or a disease spirit? What about lesser servitors? Many of them - merfolk and minotaurs, for example - are former people. Are they still people? Doesn't it matter whether the being ever lived as a person, or if they are just descended from former persons?
I have opinions on these questions, but I generally keep them to myself and leave them open questions of the setting that the PC's are free to explore and come up with their own answers. Good philosophers and indeed even the Gods of Good don't agree on the answers. The questions are just too complex and don't lend themselves to easy solutions, and even the Gods aren't omniscient.
Sadly, players are rarely serious about this sort of thing or give it much a second thought, even when I have NPC's bring up their beliefs on this and even though my reward system considers converting someone to your cause or defeating them in any other long term manner to be as rewarding mechanically as killing them, and even when they are nominally playing a good character. The more usual sort of thing is to play for a few years and then start asking themselves, "Wait a minute.... have I just been going around murdering stuff just because I could?"
At least though, because of the way I introduce and RP goblin NPCs, it's generally accepted that goblin babies are still babies. That soul searching generally happens the first time the player realizes that the 'dungeon' is really just a goblin village. The inhabitants aren't necessarily nice, but they are just basically villagers. Sadly, I've never gotten to play two long campaigns with the same group to see if the player seriously reorients their perspective during play.
