By Dave Palumbo, "Terrible Weakness":
Since not everyone seemed to be figuring it out where I found it, or they had weird theories, I played art analyst:
The way I see it, he killed a woman who loved him. Her spirit is being pulled away from this world (the knife is in his hand, her death is recent), but even in death, she continues to love him (still hugging him, not letting go). Even as his shirt is soiled with her blood (a heart-shaped stain - implying love between the killer and the victim), her peaceful expression makes it seem she has forgiven - perhaps never blamed him to begin with.
One way to look at the name of the piece is that she had a terrible weakness - her love for him. But love isn't terrible; neither is compassion and forgiveness. The real terrible weakness, I think, is his proneness to anger and violence, the inability to control it. The destruction it wrought has been awful. It ended her life, and cost him a future with a woman who loved him. She departs in peace; he stays to live with the burden, already haunting him.