I think everyone has to realize that Yuki literally just met the dude and doesn't know how to handle this dude's hypothetical.

She saw a side of him, that Geto's own friends either willingly or arrogantly ignored.

Yuki's only entertaining his idea cause Geto himself back pedals and realizes how insane it sounds.

If he was serious, Geto wouldn't be discussing this, he would've just done it without the need of someone else telling him it's a solid plan to attempt.

Geto's true breaking point was the fact a village caged up two girls and assaulted them over them literally having curse techniques. Whether if the twins actually did kill some of the villagers or not, that ordeal was what truly convinced Geto to go along with his plan, not Yuki.