why would he just now leave a tarot card!....I would think that he would have been leaving cards all along, taunting the LE.
That's my thought, too. Why now, after 6 dead?
Because he's toying with the police and the media. Numerous TV and radio shows had "expert" profilers on, over the weekend, and a common theme was that they felt the killings gave the killer a feeling that he is God. That he gets a power rush from deciding who gets to live and who dies. I'm not sure that this is true for this particular killer. I think that tarot card and the message are just another case of him giving the media what they want.
When it was reported that authorities thought he was moving out of the area, he drove down to Fredricksburg (1.5 hour away) to make it look like he was moving south. When it was announced that they were concerned that about the schools, he made his next attack against school children. At the same time, they report that he might have a feeling of being God, and this card and message turn up. I don't think its a message or a signature but just another piece of a perverse game he's playing with everyone.