As the start of his introduction, it's shown he's had some problem with Error Weegee. But it was never explained why on the first episode he was brought on. He goes to AGK's home planet, every decade. He's a god of the cosmos, and wants real respect from everyone he meets. He also likes to try different foods, and it scared Mario enough to give him a bagel.
No seriously, he crazy demands food. He went in a road, and demanded each person who drives by to give him food, or they can't pass. Leonidas hit him with his car. It triggered him enough to destroy the city. AGK was willing to stop him, but the issue he's too powerful to be stopped. To put a stop to him, AGK kicked him into his own Portal. It worked, cause he didn't notice. It's also important to note, he can get out of the Simulation anytime he wants too.
Wait A Moment
He managed to kick back from what AGK did, and forgot completely about Giganeegee. He decided to spare Earth because of Error Weegee. Turns out Error Weegee destroyed his old planet and army. It bugs him how Error Weegee brags he will be top spot. Before giving anymore information to AGK, Negative Luigi uses his "Gray Ball" to freeze Lord Beerus.