Well goblins are immortal (in the sense that they don't age), so no matter how much time passed there could be "one of the first of his kind" ones. If they were conquered early on, then they'd become permanent members of the dwarven civilization. Since goblins are immortal they can advance very high in society, permanently biding their time in a current role while waiting for dwarves to die off so they can get promoted. Plus, they have much more time to gain social skills than dwarves, which helps to gain advancements. So it shouldn't be surprising that conquered goblins end up taking over many important jobs in the civilization.
Not that is not how it works. Unmodded dwarves are entirely defensive in world gen, in order for them to go on the offensive you have to give them siege criteria in their raws (which in Forgotten Realms Mod I have done). That means that goblins cannot be added to dwarf civilization by conquest because dwarves do not conquer anything.
They actually immigrate peacefully into dwarf societies. They do this a lot because the goblin entity tolerates the cave_detailed, which means dwarf settlements. The reason there are so many goblins is that goblins have very large settlements (up to 10,000) and population growth is exponential, so the bigger the population the faster it grows.
When the population reaches between it's soft population limit and it's hard population limit, members of their civilization start looking to migrate to other settlements that are under their soft population limit. Since goblins are generally on the offensive and dwarves on the defensive, the goblin settlements which they prefer are generally full up they end up moving to new dwarf settlements or settlements that have been reduced by assaults by monsters of other civilizations (such as the goblins themselves).
Once they are there there is no getting rid of them, even if all settlements have been full up for centuries the goblins that originally migrated early in the game are still there. Also baby snatcher has an effect, emptying dwarf settlements of inhabitents and adding to the population of goblins, thus causing them to migrate into dwarf settlements to essentially replace the snatched.
I know if I had a goblin liaison I would consider killing it, even if part of my entity.
Also, racism in DF and fantasy worlds isn't the same as racism in real life. IRL there is only one race in the fantasy sense, we are not different, or only by culture and only slightly by genes.
In Dwarf Fortress, goblins are inherently cruel sociopaths. It's written in their raws.
Of course this particular goblin has dwarf ethics, which is why I am actually being unsure about kiling it, but he still has a goblin personality.
If I say in Dwarf Fortress "Goblins are evil, sadistic, hateful creatures and I want nothing to do with them" it's mostly true. If you say in real life "[insert human group/ethny here] are evil, sadistic, hateful creatures and I want nothing to do with them" it's serious business (not to mention bigoted) and you can go to jail for that.
Actually what you say about goblin raws is not true. Goblins have a baseline cruelty of 50, meaning that the only true statement about them is that no goblin is less cruel than the average dwarf is. To say that goblins are inherantly cruel psychopaths is not a truth at all.
This is the fundermental fallacy of racism, it may be true that group X is on average such and such more inclined towards such a status. There is no such thing as Mr. Average and since nobody is Mr. Average no actual conclusion follows in dealing with anyone at all.
That means that fantastic racism is still racism and racism is actually wrong regardless of facts about racial differences; aside from the situation where an entire population is completely identical what is described in the above paragraph is true.
Since racism does not exist in the game world and everyone seems to understand the above, it means that it would actually be out of character to kill the outpost liason for being a goblin.