I read Nana Yuri a while ago which was before this chapter was released, and just reading this now has surprised me since the chapters before have been all cutesy, and this one is taking a dark turn
My impression is that the author is calling back on older class S yuri, where the idea of double suicide as an act of love is more common (see Oniisama e). It just feels particularly odd because of how it clashes with the otherwise cute tone and artstyle.
If this author is crying out for help, I'm sure changing artstyle isn't easy for anyone