Why do people hurt you when you're the most vulnerable?

I went on iChat to talk to my friend -- well, vent to her -- and then this rude girl who demands respect from everyone messaged me while I was online and said, "Doug, you're so fxcked up. You don't deserve help, because you made yourself like this. You're a loser and a screw-up...do [the name of my school] a favor and leave. You're giving us a bad name." She basically alluded to my depression, my suicide attempt and basically everything bad that has happened in my life recently. The other day, she said that if my mother -- who ped away when I was nine -- were still alive, I wouldn't be a rude jack---. I was just wondering after I read the message she sent me...why do people hurt you when you're the most vulnerable?