Freiza comes to earth himself after his forces are defeated by the hero association. He thinks 'Hm. Maybe I can have some fun with whoever took out the Ginyu force. They may become useful recruits.' But once he gets noticed some heroes attack, but Freiza kills them in secconds still in 1st form. After that he smirks and searches for more fights, deciding to have some fun before turning earth into another nameless planet under his empire.
Meanwhile just 1 block away
Saitama was enjoying his day and relaxing. But he hears the sound of someone getting beat up outside. Sonic just got murdered by Freiza. Saitama jumps out his window and runs over.