Well nowadays I like to offend people. Like the girl at work, I've only done two shifts with her yet I'm making offensive jokes left right and center. Pretty sure she went home crying that night. Yet I didn't feel an ounce of sympathy (or is it empathy?) for her. I just like to push the boundaries with people I interect with yet don't really care about. I have no idea if that the same with you OP, but I guess I never used to be like this and life being boring has forced to me to find some enjoyment at the expense of others.
The lack of sympathy/empathy, I believe that might be it. I used to be very, very sensitive and I don't know when I lost the ability to be empathic/sympathic but I pretend that I am. I fool people to believe that I care about them when I don't even care about myself.