Ponyboy observes that Dally simply could not handle the news of Johnny's death:
"Dallas is gone. [...] He ran out like the devil was after him. He's gonna blow up. He couldn't take it" (152).
In this moment, Ponyboy also realizes that Johnny "was the only thing Dally loved" (152). All of the boys who were at Pony's house at the time-- Darry, Soda, Two-Bit-- seem to agree quietly that Johnny's death may have finally broken Dally's tough exterior. At the end of this scene, Darry answers the phone, only to hear from Dally that he has robbed a grocery store. He agrees to meet the boys at the lot, so they can help hide him from the police.
At the lot, Dally raises his gun, and the cops open fire. Ponyboy realizes "that was what Dally wanted [...] because Dally Winston wanted to be dead and he always got what he wanted" (154).
By robbing a story and holding a gun in front of the police.