Lady Macbeth - one of literature's vilest creatures - has blood on her hands. Metaphorically, when she pressures her husband to commit Murder, then literally when she smears Duncan's blood on the guards to inculpate them in the crime. But after the deed the dark lady is haunted by the blood. She imagines it on her hand, and it cannot be wiped away. She cries "Out, damned spot! out, I say!"
Likewise another vile creature, one Joe O'Reilly, could not erase the blood of his hands. The tap he left running at the scene of the crime was his was of crying "out, damned spot". Not only that, he was haunted and obsessed by poor Rachel's blood, which he had so cruelly spilled. Weeks after forensics had left, he hadn't cleaned the blood. And he would return to it again and again with his vistors, including Rachel's parents, God help them, who would be shocked and terrified as he re-enacted the murder, in an uncanny, trance-like manner. Blow by blow, spat by spat of blood.
Lady Macbeth ultimately goes insane and dies of her own hand (we think!). The blood simply wouldn't go away. And neither would it for our contemporary psychopath, O'Reilly, who will now have ample time to consider the wisdom of his cruel, horrendous crime.