This one is pretty simple, but hard to get away with. It spills blood, and if you don't do it right your victim might not die. Be sure to cut arteries, and if you slit the throat make sure that they are fully dead before you leave them.

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The sky was an inky black color as I walked through the woods, dragging the girl who wronged me for the last time by the hair. She took everything from me. My friends, My grades, my popularity. She had to pay. I hummed happily, almost skipping as I held a sadistic smile.
I finally got to where I was going. A tall, old house stood in front of me, and I flicked on the porch light but it did nothing, the blub dead. I groaned in plain annoyance, and I felt slight shuffling from the girl I was dragging. 'She must be waking up!' I though excitedly but I knew I had to hurry. I picked her up bridal style and went inside, walking down the basement steps. In seconds she was tied up to my wall with chains, and I hummed happily, going back upstairs to grab an apple to eat. I made my way back down the stairs when I heard a high-pitched scream. Rolling my eyes, I walked down, putting a gag over her mouth. The girl looked at me with bloodshot eyes, tearing running down her face as muffled cries escaped her body. "Shhhhh, you're getting annoying~" I cooed, snickering after.
I leaned over and picked up my trusty knife, now a bit rusted from years of use. I put it against her cheek and made a tiny cut, watching the crimson blood dribble down. Her muffled screaming got loud and I wiped her putrid blood off my beautiful knife as she struggled against her chains.
I flicked my knife a bit as I put the tip against her throat "I could kill you in any number of ways..." I hummed "I could slit your throat..." I trailed the knife down to her wrist "I could cut an artery..." I put it to her chest, where her heart was "I could even stab your heart."
Her eyes went wide and I smiled "Bye, honey." Before I reached up and slit her throat deep before stepping back and watching the blood spill as she gurgled on her blood, watching it spill out and spray on my shirt. Right before I knew she would stop bleeding I plunged the knife into her heart and kept it there until it stopped beating.
Stepping back and grabbing my knife, I  looked at my work of art with a smile. She didn't last too long, but there was others to come.