It's most definitely Seinen.
It's just too obvious... and Seinen mangas/animes aren't always adult manga but more of that gap between Shonen and Adult mangas/anime, and no Hentai is beyond this level... and most are really just having lousy plots and troups thus a majority are just in the back seat...
I can see some people getting confused with the term Seinen and how that manga and anime weren't really being called this not solely based on their artstyle (Since each artist has different style in animes, saying them look the same isn't that general and even narrow minded to some extent) but rather their themes and the subject and topics in them...
So I am really not that surprised with how Goblin Slayer episode one went with if it is Seinen, not to mention being outraged by it. Then again I'm not saying that all Seinen manga/anime have this disturbing or close to R/15+ content... there are many Seinen mangas that are kid friendly or funny that just have some more serious topics.
This might be a hot take to some people but I have been enjoying more of Seinen manga/anime more than Shonen manga/anime, like Space Brothers, One Punch Man, Hanebado, Megalo Box, and etc... last but not least my most favorite series of all... Made In Abyss and yes... this is a Seinen manga/anime there's no debate, it's cute looking but this basically involves with an abyss that almost everything kills you...
If Goblin Slayer is too dark for your taste... try Made in Abyss
If Made in Abyss is still too dark... go check out Hakumei and Mikochi... this got an anime adaption this year which is literally kid friendly version of Made In Abyss or "Animal Crossing anime with better story".