Why did Dally rob a grocery store?

Dally has a death wish after Johnny dies. He robs the grocery store to get the attention of the police. Dally plans for the police to kill him; Ponyboy knows he's only bluffing when he pulls out his gun, but the goal is for the police to shoot back. His death makes Ponyboy realize that although Johnny appears a hero while Dally appears a hoodlum, Dally was heroic, too.