Responses
It's plot convenience so Sangwoo would have nothing in the way of his hands when he tried to scoop the blood back into Bum's body later. Nah, just kidding （⌒▽⌒）. Either Sangwoo put it back somewhere after the "okay, die then" part or it was an oversight by the author.
ok, kkkk
On pages 22 and 23, the knife is there, but on page 26 it no longer appears. Was it the designer's mistake or something important in the plot? Maybe it's nothing.