One chapter with a single takeaway of a villainous monster doing something villainous and monstrous is no reason to flag an entire manga as having "sexual violence," Just like a single chapter of gender bending shenanigans in a fantasy manga isn't enough to get the entire manga flagged as such. To suggest otherwise is absolute madness.
No, just because you really hate the villainous and monstrous thing does not change this.
The site does not have the ability to flag individual chapters for tags, so this is as far as it can go really. Having said that, I rather like the idea of individual chapter tags, and maybe some way of highlighting chapters that have certain tags, like manga that aren't flagged H or ecchi but chapters that ARE being in bold or red or what have you, so you know not to read that chapter of Funny Fantasy Manga #512 on the bus, or someone who really doesn't want to see sexual violence can get a warning when opening the chapter.
Ultimately though, this seems like the sort of thing that asking translators to put in content warnings or -- ugh -- trigger warnings at the start of chapters might be the way to go. Failing that, maybe some mix of hardening up and seeking therapy on the part of the reader is warranted, especially if they are either having some form of metal breakdown at the idea of bad people doing bad things in fiction, or are pretending to have one for clout and attention.