If she did that, no one would care besides Uncle Bill and maybe a few other servants.
The part where Mathias brings up his father and grandfather wasn’t clearly explained in the manhwa, but in the novel, what he means by bringing them up is that both his father and grandfather had many mistresses. This was very obvious to the family members because they never cared about hiding it, including from his mother. No one cared about their infidelities during their lifetime, and no one cares about them even after their deaths. Everyone remembers them for their status and achievements, while their mistresses won’t be remembered.
That’s what Layla meant when she said she wasn’t enough to leave a stain on this man.
I'm sure Uncle Bill would rather take responsibility than have her do this. I wish she would just out him to his family, fiancee, staff, the town, the world, and just everyone. Sick son of a bitch deserves to live in ridicule and shame. It's sad that I'd rather see her write a detailed letter on his bs and publicly off herself than submit to his will. This shit working the fuck outta my nerves but I need to see what's gonna go down in the end—depicting this with art this beautiful should be a crime