why did dally kinda kill himself?

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. The appearance of the whole situation is much like the Greasers' appearance in general: misleading.

Ponyboy links Johnny and Dally's deaths to Gone with the Wind, as he considers how they "died gallant." When Darry tells him that Johnny has left him his copy of the book, Ponyboy can only think of "Southern gentlemen with big black eyes in blue jeans and T-shirts, Southern gentlemen crumpling under street lights."