The following article contains spoilers from Action Comics #1042, on sale now.
Superman's revolution on Warworld isn't so much a war against Mongul as it is a war for the soul of the planet. He wants to liberate its people and show them that they are more than just Mongul's playthings. In Action Comics #1042 (by Phillip Kennedy Johnson, Riccardo Federici, Lee Loughgridge, Dave Sharpe), Superman even told Mongul that the people of Warworld are stronger than they'll ever be because they survived this planet's horrors without the strength that Superman and Mongul wield on a daily basis.
In his eyes, that makes these people "more" than him or Mongul. While meant to lift his people up and also to tear Mongul down, there is a darker side to this belief. If Superman believes that the ordinary person is better than him, then he also believes that he is less than they are. If so, then this suggests that Superman has some self-worth issues, perhaps even explaining some of his recent behavior.
Superman has always been an inspiration to the DC Universe. He represents everything good about humanity and is a beacon of hope for the future, inspiring countless others to follow his example. His latest battle against Mongul has pushed him to his limits and brought out both his best and now apparently, his worst. The idea that he is less than the ordinary person suggests that Superman does not place much value on his own life.
This only seems to be supported by his decision to go to Warworld when he knew his powers were weakening. His battle with O.M.A.C. in the same issue had him explain that he couldn't stand by and watch while others suffered at Mongul's hands just because he wasn't at his best. He believes it is his duty to fight for the helpless even if he isn't the strongest.
While this is undoubtedly true and an extremely noble part of his character, it may also be motivated by a lack of care for his own life. Since he has been on Warworld, Superman has thrown himself into dangerous situation after dangerous situation, almost dying a number of times because he believes that the lives of those he protects are worth more than his.
If the likes of Batman or the Justice League call him one of the best people they know, then surely his life has tremendous worth. Superman's own words seem to suggest that it is his powers that make him less than those around him. This actually makes sense in a way. His abilities have enabled him to challenge gods to protect the world he loves, but it also alienates him from those around him. That separation from humanity might lead him to believe his only worth can be found as a protector, to keep these fragile, but more valuable lives safe and prevent them from going through the same extinction his kind once did.
It's even possible that he believes that even if he was human, he wouldn't be a productive member of society. The idea of Superman without powers is a fascinating one, and has been explored from time to time, but with this new insight into his character, it's possible he may just feel his only worth is as a guardian.
