What was the moral when Darry fought his ex-best friend Paul Holden in The Outsiders?
I think the moral is quite clearly stated by Ponyboy in Chapter Nine of this excellent novel, which narrates the rumble and how Darry and Paul Holden begin to engage each other in a fight. We are given their history and how Paul Holden and Darry used to be best friends, but now, Ponyboy points out how ridiculous it is that they are enemies just because of circumstance. Note what Ponyboy tells us as they begin to circle each other:
Even I could feel their hatred. They used to be buddies, I thought, they used to be friends, and now they hate each otehr because one has to work for a living and the other comes from the West Side. They shouldn't hate each... I don't hate the Socs any more... they shouldn't hate...
It is clear therefore that Paul and Darry are an example of the completely unreasonable hatred that exists between the two groups in this novel that only exists because of circumstance rather than for any other more meaningful reason. Clearly, Paul and Darry in an ideal world still should be best friends, but the poverty of Darry and what happened to his parents has taken him out of the world of the Socs and now they stand, irrationally, on opposing lines, fighting each other.
I think the moral that the "bigger they come, the harder they fall" could also be applied here. It could be argued that this matchup at the rumble was a case where the biggest, fastest and best educated man does not always have the edge over a lesser man. Paul Holden was "husky" and probably much larger than the muscular yet lean Darry. A star halfback in high school, Paul was now in college with a bright future ahead of him; Darry's future probably would remain in roofing or construction. Although we don't know how their individual matchup turned out, we can assume that Darry got the best of Paul. After being distracted by Dally's sudden appearance, Darry took a "hard right that would have felled anyone" else. Darry still had time to watch Pony during the fight, "keeping an eye out for me." Paul eventually ran away with the other Socs, leaving Darry and the greasers as the clear winners.