Looking for an old manga about a boy and a girl investigating freaky murders while also maybe being serial killers. The art style was very sharp and detailed. Not at all cartoony by manga standards.

Notable scenes were frames of a decapitated woman holding her head in her hands, and another character (maybe MC) getting buried alive in a box with a straw sticking out above ground. Butterflies were a big part of the symbolism iirc. I think maybe the boy protected the girl pretty often but only because he wanted to kill her himself. They investigated the murders together and I think helped solve them, though definitely below board if so.

I feel like a psycho searching for it but I just got flashbacks of reading it when I was a kid and it's killing me not knowing what it is. It was a haunting read.

Pic is of a vaguely similar style.