Omni-man left because when he hit Invincible one final time to push his son into the air, he realized that he had never really felt proud about him.
To Viltru, seeing a child play is akin to wasting time; however, it’s the way that adults in his planet feel about their children.
This memory definitely changed him. It reminded him of how much he loved his son. It also changed him. He realized how much Earth has changed him.
It is extremely peculiar that the Omni Man, who is just a mere Viltrumite, could be left behind to conquer Earth. After the Omni Man did such a terrible thing back on Viltrum by killing children and killing thousands and killing animals, how could he possibly decide that he wants to conquer people? It is much more likely however that the Omni Man did that out of his own humanity. Mark was his son and perhaps some of the Omni Man felt what it was like to be human as he killed thousands of people.
What is the omni man’s ultimate goal?
The omni man’s ultimate goal is to destroy all life in the universe. He wants to see everything and everyone gone, and he will stop at nothing to achieve this goal.