- Intentional or not, Robert Kirkman's Invincible series shows why a secret identity is vital to being a superhero. Mark starts out as a typical superhero, having a day job, a normal human girlfriend, normal friends, going to school, etc. But over the course of the series:
- Rather than juggle college and being a superhero, Mark simply quits school. He also quits his day job and starts working directly for his universe's equivalent of SHIELD. He later gets a job essentially working as a Hero For Hire. As a result, Mark has no real interactions with normal human co-workers, never develops a non-superhero supporting cast, and his only interactions become other superheroes and aliens.
- As a result of not maintaining any ties to normal people, Mark eventually breaks up with his normal girlfriend and dates Atom Eve exclusively. His also gradually stops interacting with his only normal friend William. The two get phased out of his life, and he never makes any real effort to reintegrate them into his life.
- As a result of losing his connection with normal people, Mark loses the perspective of a normal man. This allows him to go down some fairly dark paths and to make some incredibly poor decisions, such as a willing to murder as a quick solution, deciding to help Dinosaurus, being okay with simply abandoning Earth several times, etc. Mark finds it easy to lose a human perspective because he loses anything to ground him in a normal human life.
- As a result of his job under Cecil, Mark gets used to taking orders from people in authority, and gets used to following other people's directions, including Dinosaurus, which leads to disaster. And when that trust gets betrayed, Mark tends to react badly, lashing out angrily at those in authority and making the situation worse. He never really figures out how to be a solo hero, but also never really figures out how to work well with others. He winds up becoming an okay follower, a sub-par team-player and poor solo operator. It takes him years of effort during a time-skip to eventually become a decent leader.
- Because he never really learns to work on his own, Mark never really developed the kind of cleverness most heroes develop over a similar career. He tends to rely on brute force to win most of his fights, with the occasional attempts at diplomacy. He never really develops any brick tricks or how to make use of his powers beyond simply punching the thing in front of him to death.
- Thanks to his over-reliance on brute force, Mark never really learns to plan ahead, consider the consequences of his actions, and continually makes very poor decisions. He constantly does things that end with him stranded in other places for months or years, and never really matures as a person. It takes becoming the leader of his people to really make him grow up.
- Because of his lack of human connection, Mark eventually becomes disenchanted with the whole superhero life-style, frequently decrying it as pointless. While never malicious towards normal people and not wanting people to get hurt, he becomes far more focused on things that matter to him personally, such as his family, threats to them, and the Viltrumite people. That doesn't make him a bad person, as families should be a priority, but he makes no effort to try and balance being a hero and being a family man. He just essentially stops caring about trying to use his powers to actively help others.
- Ironically, while being with humanity teaches the Viltrumites how to connect to other people, to care about others and grow emotionally attached to non-Viltrumites, Mark slowly loses his connection to people outside of his family and his species. The Viltrumites grow closer to humanity while Mark grows more distant from them.
- Put simply, intentional or not, Kirkman shows Mark to be a decent person who grows up into a good leader, but is ultimately a terrible superhero. Since he slowly loses his connection to humanity as a whole, he focuses instead on people who matter to him personally, namely his family and the Viltrumites. Superheroes are generally meant to be altruistic individuals who put helping others above their personal happiness (though not to the exclusion of said happiness), and Mark just isn't cut out for that. Ultimately, he prioritizes the well-being of certain people over others, which is something a superhero cannot do.
- At the end of the series, Mark decides to destroy Robot's body and gives the Immortal effective rulership of the world with Robot's brain to act as advisor. However Mark already went to the future at an earlier point to discover the Immortal has gone insane from his long leadership over Earth after Mark left and has become a suicidal tyrant launching ritual genocides in the hopes that someone will kill him at last, which Mark finally does. This is essentially a time loop. Why on Earth would Mark choose to make the Immortal the Earth's ruler knowing how badly it would turn out?
- We can infer that it's not the same timeline, though. Immortal said that Mark took his family and left the planet. But Mark's son, Marky, is still on the planet in the final issue's timeline. Likewise, it's implied that the Earth has started to venture into the galactic community. Additionally, the Immortal's descent into madness happened several hundred years after a "great disaster." Exactly the sort of thing that Mark's new Viltrumite empire would intercede in. That said, the Immortal was also forgetting things and had gone insane, so he might have for example, confused Mark with Marky, and the whole bad timeline could be even further off in the future than we suspect.