A controversial place to start no doubt, but retrospectively, the decision to give the killer of Bruce Wayne's parents a name, a story and in one memorable case, a death was a mistake - not a ruining one by any means, but a mistake all the same.
Yes, it was an established storyline that ran for many years, and shaped a lot of things in the character's make-up, but events that came later proved exactly how good the alternative could have been. Chill only really became a mistake when he was retconned out of existence briefly with the events of Zero Hour, when Hal Jordan effectively gave DC an opportunity to rewrite any origins they wanted to play about with, that weren't changed in Crisis On Infinite Earths.
After Zero Hour, it became canon that the killer of Wayne's parents was never identified, and the anonymity helped make him an even more enduring symbol to Batman's cause. That decision robbed Batman of a specific focus for his grief or his bubbling rage, which added far more poignancy to Batman's endless torture, until the retcon was retconned again, and Chill got his name back.