Patsy Parisi has always hated Tony, for the murder of his twin brother. In one episode he even tries to shoot him at his home, but runs back into the bushes without Tony ever seeing him.
After Christopher dies, there is no heir apparent, no one to avenge Tony should he be hit. Christopher was by the time he died easily the most dangerous guy in the gang. From that point on, Tony's empire is built on sand.
Christopher's death opens up a power vacuum at ground level, quickly filled by the ambitious Patrick Parisi. Patrick has started dating Meadow, and knows which restaurant they are to meet at (NOT Vesuvio).
AJ walks in with the bodyguard, who knows what is about to go down. Aj sits at the table with his family, the bodyguard sits nearby watching nervously for Meadow to arrive. When she doesn't show up, he gets up and goes into the men's room. Tony gives him a look, then shrugs 'I guess he's gotta go'.
Patsy comes out of the men's room and walks up to Tony. Tony look s up and Patsy shoots him.
He shoots AJ, he shoots Carmela, she shoots Meadow on the way out. Janice, Paulie and Uncle June are already dead.
This echos the restaurant scene in the Godfather, which Tony dreamed about in "The Test Dream".
Remember Patsy's parting words to the car remember; the last face you see won't be Tony's - it'll be mine"