1) Ex Lover's Lover by Voltaire is a song about a guy who fantasizes about killing his ex-girlfriend's new love. It's morbid, gory, and creepy. A perfect solution to Valentine's Day sappiness,
2) This is The Raveonettes' cover of I'm So Lonesome I Could Dry. It's originally a country song, so a bit of twang is retained. I love this song as a duet. The harmony and the weighted guitar strokes give it a completely different and darker feel.
3) My Eyes is a song from the epic Dr. Horrible's Sing-a-long Blog. Dr. Horrible is hopelessly smitten with Penny, but can never bring himself to speak to her. Captain Hammer, his archnemesis, saves her and they start a relationship. Dr. Horrible watches their feelings for each other develop from afar while he stews in his own misery.
4) Backstabber by the Dresden Dolls is a cheery sounding song about a backstabber. I think everyone can relate. I love it and this adorable video about the Dresden Dolls and Panic at the Disco wanting to kill each other.
5) Mad World covered by Gary Jules is the best version of this song ever. It's depressing as hell and the lyrics are better when sung slower. In this version, the music matches the sentiment. I chose this song because of one person's comment on the video: "I always listen to this song when i'm feeling down . . . So i'm going to be here a lot today since it is valentines day."