The worst and cruelest thing I've done is tell a girl the honest-to-God truth. I gave her the whole "I don't do relationships" speech. She was receptive to the playmate idea, so we went ahead with it. Fast forward 4 months, several parties, lots of sex and 2 ski trips later. We worked together, and I was unwilling to show up one particular day, and I gave her the same story I gave everyone else at work. She found out and got all pissy with me (nothing to do with another girl, she was just pissed about being lied to).
Honestly, at that point I f*&king hated her personality and even the sex sucked because I simply wasn't attracted to her. I was amazed that she had the audacity to yell at me. Now, picture the scene in "A Few Good Men" where Jack Nicholson is being grilled by Tom Cruise in the courtroom. I didn't say specifically, "YOU CAN'T HANDLE THE TRUTH" but something very similar with the same meaning. She insisted on the truth so I told her flatly, "Honestly...I'm not attracted to you and frankly, I'm embarassed to show you to my friends."
This was the truth. Her voice just cracked and then she just got all silent. At this point I was furious that she'd made me lose my cool, so I hung up.