Earth-2 Superman takes steps to ensure that Power Girl will not take part in the Final Battle. It's easy to dismiss it as sexism, until you remember that he saw Earth-1 Supergirl dying. And then he saw as her existence was erased from memory and history. Like HELL he'll let Earth-2 Kara meet the same fate, sexism or not.
In the seventh issue of Crisis on Infinite Earths, Earth-1 Superman cradles his cousin Supergirl while she dies. In the seventh issue of Infinite Crisis, Power Girl cradles her cousin Superman while he dies.
The idea of Golden-age Superman pulling a Face–Heel Turn (even if it didn't last) might seem hard to believe for those who know his reputation, but as articles like this can show Earth-Two Superman was slightly less moral than his Big Good Earth-One and New Earth counterparts.
Alex Luthor and Superboy-Prime's Face–Heel Turn also makes sense when you realize that Alex was a baby who grew up too fast (literally) and Superboy-Prime's universe got destroyed. By the end of the Crisis, both of them end up trapped in another dimension to survive with no way of escaping (and apparently any attempt to so would destroy the surviving universe, according the Who's Who guidebooks).
Many Happy Returns came out less than two years before Infinite Crisis. The four survivors of The Multiverse saw this story's events playing out. Earth-Two Superman watched how his Earth-1 cousin -whom he owed his life- was ignored by Post-Crisis Superman and hurt and mistreated by other people -who were alive thanks to her- and the human girl who took up the Supergirl's mantle got screwed-up. Undoubtedly it cemented his belief that Post-Crisis Earth could not be saved by its heroes.
can show Earth-Two Superman was slightly less moral than his Big Good Earth-One and New Earth counterparts.