can anyone rec me a manhwa or manga with the mc who just kills? like sum kind of vigilante...

https://www.mangago.zone/read-manga/reaper_of_the_drifting_moon/ ?

Well they dont just kill, tried to give some variety also they're all women so yeah
https://www.mangago.zone/read-manga/serial_killer_isekai_ni_oritatsu/
https://www.mangago.zone/read-manga/a_married_killer/
https://www.mangago.zone/read-manga/aftergod/ (not really on topic but too good not to recommend)
https://www.mangago.zone/read-manga/azumi/
https://www.mangago.zone/read-manga/claymore/

Into the Red Fog?

https://www.mangago.zone/read-manga/brutal_satsujin_kansatsukan_no_kokuhaku/