Goblin Slayer: not your average fantasy anime with a main character from another world or with a lot of comedy/harem element (like isekai or taking place within a game). Instead, this is a dark fantasy anime that sticks out with gore, violence, and brutal murder unlike most fantasy anime today.
When the first episode came out, a lot of viewers were disturbed with the first episode as a lot of them watched it without having proper knowledge of what they should have expected. Kumo Kagyu, the author of the light novel series, wanted to make a story that stood out among any other fantasy anime today; thus coming up with Goblin Slayer (anime adaptation by White Fox).
The anime starts off with The Priestess joining an adventurer guild after receiving her training with light and holy magic. She gets asked to join a small group of porcelain ranked (lowest level) adventurers to join her group to go kill some goblins in a cave, and so she accepts.
Everyone in the group seems happy, very eager to complete their quest, and truly do believe that it will be easy for them to get the job done... but this isn't a romanticized fantasy anime and things don't turn out as well as they expect it would.
The goblins made a sneak attack from behind. They overwhelm the adventurers one by one and everyone but The Priestess gets slain; one of the girls, The Fighter, gets raped (not shown but is suggested from clothes being ripped off and her screaming as a shot is shown away from her).
Just when the goblins were about to gang up on The Priestess, a man in a full set of armor arrives and slays them all; hence being the "Goblin Slayer" as said in the title of the show. And so he goes out to fulfill the quests the adventurers have failed to complete along with The Priestess helping him out with the remaining goblins.
He shows to have true understanding of goblin psychology while never hesitating for each kill made. After completing the quest, the priestess decides to join him as his adventuring companion.
Goblin Slayer Opening
Goblin Slayer | GOBLIN SLAYER
So let's bring this question up: why have such a gruesome anime filled with horror and violence? Why not have it be a much less horrific anime like Konosuba, Sword Art Online, or even No Game No Life?
In today's world, isekai and video-game-based anime are what's trending for authors and anime studios to make. Kumo Kagyu, on the other hand, wanted to reinvigorate horror fantasy anime once again as it used to be back in the old days of anime.
Kagyu talks about this in his interview hosted by TheAnimeMan on YouTube where he goes over the making of Goblin Slayer as well as his journey as an author.
Interview With "Goblin Slayer" Creator Kagyu Kumo
He also talks about why the characters don't have particular names but are labeled with who they are such as Goblin Slayer, Priestess, Guild Girl, etc. He wants the audience to decide on how to perceive these characters; basically granting them the ability to view them by personal perspectives.
Going with the anime adaptation, this is very perfect for the first group of adventurers seen and killed. Even if we, the audience, learn their names, they would eventually be forgotten. Instead, we are shown a couple of times with flashbacks in black and white but having their eyes being colored (something to differentiate all of them that have died).
Also with the nicknames comes to use for some of the characters being well known to certain peoples. A lot from other lands have heard of The Goblin Slayer's deeds in taking on quests in killing goblins. And so the elves know him as Olcborg, the dwarves as Beard-Cutter, and the lizard men as Sir Goblin Slayer.
Request | GOBLIN SLAYER
It does also bring great in depth mystery for the protagonist as we do not know his name but rather what he does and his past as we get to learn more about him through each passing episode. From first impressions, we can tell that his daily life is eat, sleep, and killing goblins with an added touch of 3D animations from time to time to show of his inclination to daily duties.
A great example would be whenever he arrives at Cow Girl's place to eat, sleep, and rest, he always checks for tracks of goblins and any evidence left behind. Say if a fence got broken down a bit, then there were goblins lurking around (perhaps he has hidden care for Cow Girl to make sure she's not targeted; a thought to pass up).
Daily Routine | GOBLIN SLAYER
The only time we don't see him as a 3D animation is whenever he's having a close/intimate conversation with others. From those conversations, his appearance can easily level with the people he's talking with and be more than just somebody who's only desire is to kill goblins but be able to have more empathy towards him.
Where do Goblins Come From? | GOBLIN SLAYER
And from having more empathy for him, we begin to understand why he must feel to only kill goblins. After all, he had a sister once and saw the horror unfold before his eyes as she was raped and brutally murdered.
It's very reasonable as to why Goblin Slayer only kills goblins not only by this scarring past he has but how other adventurers haven't been taking on these quests. The higher ranking adventurers don't go for these quests as they seem too easy for them to do.
Not only that, but the lower ranking adventurers can barely stand a chance against them; most likely die just like the first batch of adventurers. And so, I perceive him as a sort of an uncelebrated hero for solely slaying goblins where hardly anyone would be able or even want to take on these quests.
I also do appreciate that Guild Girl is happy for Goblin Slayer bravely taking on those quests as she understands how critical it is to slay as much as possible with even women being taken captive, raped, and mentally scarred.