Killing Stalking (dropped within the span of less than 15 chps bc I couldn't handle the abuse, that scene when sangwoo hit beom with a hammer and I immediately went NO.)
Another VERY traumatizing one is Nii Chan by Harada, I was only 14 back then and was tolerant of all sorts of crap, this is the only one I couldn't bear aside from KS. I'd still rate this one above KS in terms of trauma though

Can everyone share their first traumatic manga ヾ(☆▽☆)