You say you have an axe with a drop of blood on it. You hit someone with that axe and now they're going to sue you. That drop of blood on your axe is pretty incriminating, which is why they have enough of a case to sue you.
You want to sue them back so you clean the axe with some soap (そ). "Your honor," you say in court, "I don't see any drops of blood, I countersue!"