What story is this?? A samurai assassin [seme] stabs the wrong guy [uke]. It's raining, and the uke invites him in for tea or something. Later on, seme visits the uke's house on the pretense of returning his umbrella. They start to fall in love.
It's later revealed that the seme's organization is trying to assassinate this other samurai group called bafuku (or something). The uke is part of the bafuku, and was indeed the correct target (uke was waiting for his leader to return). Seme is sent to assassinate him again, but can't do it. He confesses to the uke, and says he'd rather die than kill the guy. They have sex. Next morning, uke commits sepuku to save seme from retaliation by the group.
This is not Hybrid Child

It should be chapter 6 from So Cute?

It was, thank you!!