Why does Dally ultimately decide to rob the grocery store in Chapter 10 of the Outsiders?

After Johnny dies in a fire, Dally starts to develop something of a death-wish. He's so upset by his friend's death he doesn't know how to handle it. By holding up the grocery store he plans to attract the attention of the police, whom he hopes will shoot him dead. In modern-day terms, this is a phenomenon known colloquially as "death by cop." This is where suicidal individuals, unable or unwilling to kill themselves, deliberately provoke police officers into doing the job for them. Dally's gun is unloaded, which indicates that he has no intention of harming anyone; he simply wants the cops to shoot him dead.
Dally is successful—if you want to call it that—in his endeavor as he's shot and killed by the cops as planned. Like Johnny, Dally achieves some kind of redemption in his death. Ponyboy for one thinks that Dally's actions were heroic and showed him to be so much more than just a hoodlum.