Even after quickly defeating Vegeta, they had exposed too much energy and just enough energy for Buu to revive. Goku left Vegeta knocked out as he sensed the enormous power level of Majin Buu, which he goes to deal with. Even after defeating Vegeta so quickly, Goku exhausted a lot of energy and through the midst of transforming. This resorts him back to his regular Super Saiyan form as he tries to take Buu on and ultimately, loses. Before Buu can kill Goku, Buu's childish instinct allows Goku more time to train in order to 'play' more. As Babidi orders him to kill however, Buu refuses and ends up killing Babidi. Goku promised Buu that a powerful warrior would be able to challenge Buu and give him the fight he desires, speaking of course of Gotenks, to which Buu accepts.