Immortal Days - it's not horror really but it's sci-fi action and really interesting! There's not a lot of chapters here but it's free to read on Lez-hin.
https://www.mangago.me/read-manga/zombie_world/
You've probably heard of Ito Junji's works but I recommend all these author's works if you haven't
His most famous work: https://www.mangago.me/read-manga/uzumaki/
https://www.mangago.me/read-manga/jinrou_game/ (has 3 parts)
https://www.mangago.me/read-manga/300_beats_per_second/
https://www.mangago.me/read-manga/kaiju_no_8/
https://www.mangago.me/read-manga/itou_kun_wa_koi_wo_shiranai/
https://www.mangago.me/read-manga/badluck/
https://www.mangago.me/read-manga/shuumatsu_no_laughter/
I'm late but thanks everyone!
Can I get some recommendations? Genre doesn't matter, though I prefer horror and I don't like smut.