The superheroes of The Guardians of the Globe embrace Omni-Man happily, so why does Omni-Man kill his fellow superheroes? Invincible Issue 7 reveals the reason for his death.
We find out that Omni-Man is not the hero he made himself out to be. He was evil all along, but purposefully created a distraction called Earth’s protector to help his race of aliens conquer the planet later. When another group or creature threatened to do the same, he had to stop them.
This may seem harsh, but remember that they were trying to conquer the planet Earth and creating some sort of deterrence against this was necessary. Using that as a pretense, he killed the Guardians in Issue 11 because they were protecting Earth and they were too much competition for him; this weakened earth's defenses.
He did this after realizing his son was developing powers because he had waited so long and couldn't afford any more time than necessary to pass before doing something about it.
Many members of the Guardians of the Globe resemble familiar superheroes, such as a giant fish that resembles Aquaman. The others resemble The Flash, Batman, Wonder Woman, Martian Manhunter, Green Lantern, and others.
For you, it might be hard to believe that no one could stop him from getting away with the murder of the Guardians. But in Issue 12, Invincible fights Omni-Man. It is jarring when an offspring has to take on his father in a fight like this - but unfortunately, with his powers not fully developed and Omni-Man's sword slicing close to Invincible's throat, Invincibility was beaten to near death.