After the final rumble Ponyboy and Dally go to the hospital to tell Johnny that the "greasers" won the fight. When they get into the room. "Dally looked at him (Johnny) for a second, then put the knife back in his pocket. We both went into Johnny's room, standing there for a second....When Johnny dies, "Dally swallowed and reached over to push Johnny's hair back. "Never could keep that hair back... that's what you get.." Whirling suddenly, he slammed back against the wall. His face contracted in agony and sweat steamed down his face. "Damnit, Johnny.." don't die Johnny, please don't die..." He suddenly bolted through the door and down the hall. Dally feels guilty because he sent the boys to Jay mountain, he went to pick them up and he took them back to the church. When it was on fire, he couldn't get Johnny out in time to keep the beam from breaking Johnny's back. He feels responsible for the events that lead to Johnny's death. "Having lost the one thing he really cared about, Dally sets himself up for death. After robbing a store, he threatens the pursuing cops with an empty gun. He dies in front of his friends in a hail of bullets. "
I don't want to spoil anything to you, but I kind of have to. So after I said that, Dally felt responsible for Johnny's death since Dally only cared about him. Just like if your dog died you'd be miserable and blame it on yourself. He was so miserable to the point he pulled a gun after robbing a store. Making him getting shot by the cops. Which meant he suicided since he planned the whole thing. Proof that he planned the whole thing is he pulled the 'unloaded' gun out in front of the other greasers in the vacant parking lot. Only IF he doesn't pull the gun they would've been able to escape the tragedy.