Although some people may believe that Dally died a hero's or martyr's death, his death is more akin to a suicide due to the guilt he felt about losing Johnny.
Dallas is for sure a great character. He is a "brother" to many of the greasers and is always there to lend a helping hand and pull them out of trouble. Ponyboy especially looks up to Dallas.
However, there is nothing heroic about his death. He planned to die and pulled it off, but there is nothing heroic about intentionally getting shot. Maybe it would be if he was saving the others from the same death, but that isn't the case. Dallas left with a pistol (unloaded), robbed a grocery store, got caught by a police officer, pulled out his gun, aimed it at the officer, and died in a hail of gunfire. Even Ponyboy admits to the intentional death when he says:
"He was dead before he hit the ground. But I knew that was what he wanted, even as the lot echoed with the cracks of the shots, even as I begged silently – Please not him […] I knew he would be dead because Dallas Winston wanted to be dead and he always got what he wanted."