If a goblin knows how to say "I surrender, don't hurt me," in common, it probably learned that from a victim that it then killed and ate (or killed by eating).
My, aren't we judgemental. Of course, the Goblin is not human, so we can decide they are hardwired for evil. In such case, it hardly seems unreasonable to put them down. But, if they are truly hardwired to that behaviour, it seems they are more neutral than evil, in that they lack the capacity for moral choice. They still need to be put down for the safety of others, though.
I prefer goblins with free will.
p 166 of the basic rules tells us good implies respect for life. Killing is, then, not a choice to be made on a whim, or without making every effort to find other options. Those options may not be convenience, but "Goopd characters make personal sacrifices to help others".
Evil, from the same page, implies killing others. Creatures who "have no compassion" and "kill without qualms if doing so is convenient" are explicitly noted as evil. Those who "have compunctions about killing the innocent, but may lack the commitment to make sacrifces to protect or help others" are neutral.
The question asked for RAW. Those are the RAW.
Good isn't stupid. There may well be situations where killing is the only choice, and accepting surrender just to be stabbed in the back on a recurrent basis is not required. But Good is also not about taking the path of least resistance, it's not always easy, and it's not about personal convenience.
My group handles this sort of thing as "a Paladin can refuse to accept a creature's surrender."
While goblins are free-willed, a normal goblin is raised to view nearly anything that isn't a goblin as prey.
In other words, if you facing something that views babies of your species as food, your paladin almost certainly has active authority to put it down. Co-existence is so remote an option that it might as well not exist.
That sounds EXACTLY like the citizens of that country we're always in border skirmishes with, so I guess we can put them down with impunity as well, right?
Actually killing helpless goblin babies is more complicated. They can be functional if they're brought up in a sane society that spare the resources to raise a bunch of midget pyromaniacs. But putting them down probably isn't any worse an act then putting down a litter of skunks born under your house. It's a distasteful task, but sometimes that just how it pans out.
Skunks are not sentient and lack the capacity to make a moral choice. That makes it different.
A society that is Good will "make sacrifices to help others", which means they will tighten their belts and make those resources available, not decide they're "just bad blood" and slaughter them without a second thought. This is, at best Neutral - not willing to make personal sacrifices to help others.
Now, actual evil outsiders are, in essence, malicious spirits given flesh and form. Accepting surrender from a demon has about as much meaning as accepting surrender from a rabid dog.
Outsiders follow a whole different set of rules. They are hard coded Evil, so they can reasonably be taken down. If, in your game, Goblins are the same, the same rules should apply. But if Goblins are hard wired Evil, we may as well hard wire the humanoid races Good while we're at it. Why have any deviation? What makes some races hard wired and others not, on this plane?
Lawful characters judge their actions by an external, consistent standard. Depicting what they set of ethics is would help it be clearer if their actions were lawful or good.
Lawful and Good are separate. Killing for an evil master is explicitly noted as evil. Paladins don't get to "just follow orders" - they must ensure their orders are consistent with Good.
Welcome to a topic that has been debated for around 40 years in RPGS (and thousands of years in philosophy)
Yup
The short answer is really "it depends on the game you're playing" games can vary between
"game of hats" to "extremely complicated shaded morality"
In a game of hats game then unless there's a reason to believe that a particular goblin isn't evil you can probably hack it down with a free concience.
Again, yup - "hats" - "hardwired alignment".
Also note that you only detect as evil if you've got a link to an evil force of the universe or you've got significant personal power (5HD or more) by the rules a goblin could be intending to prepare Human Baby Tartare and still wouldn't detect as evil.
Revisiting p 266 - 267, while "Creatures with actively evil intent count as evil creatures for the purpose of this spell", only creatires with 5+ HD actually have an aura of evil, so my previous comments are not accurate. I am inclined to consider that the "presence or absence of evil" is still detected (otherwise, that poor paladin has no actual ability for several levels, so why should he have it from L1 in the first place), but there will not be "evil auras" for the 2nd and 3rd round effects of the spell.
That makes killing them convenient, it doesn't mean that killing them isn't evil
Bingo!