Sniper not a pro
I'm not an expert, but I don't think the DC sniper is a professional, i.e. somebody with extensive training and/or experience. Some of his actions just don't jibe:
He approached within 40 feet of his last target, risking needless exposure. The area is highly populated, making concealment difficult at best. Since he chooses his targets at random, the sniper could easily have picked another target at a more remote location, one where his chances of being seen would be reduced significantly.
If the tarot card was left by the sniper, it would be out of character for a professional, who would maintain his anonymity. He seems to be leading with his ego now, almost daring the police to catch him.