This can't end well. But you'd think Hanyuu would have learned a long time ago what happens when she hangs around crazy people in this series.

Keiichi'd her horns...

I think this resembles a little Oninakushi-hen don't you think? Or maybe another world?
And yes, this seems to be yet another world with no happy end =/.

Seems like some variation on Onikakushi-hen except with both Rena and Keiichi going nuts, unless this is supposed to be from Keiichi's perspective... which somehow I don't think it is, seeing as Hanyuu is visible here.

Also I was thinking it could be Tsukiotoshi-hen Without Shion of course,since both Keiichi and Rena show signs of Hinamizawa's Syndrome L5

They're just coming home from baseball to the butcher so they can get some meat to make tacos later.