Goku was sparing Vegeta's feelings and lying in the entirety of the saga. He actually hates the Earth and that's why he trusted its fate on SSJ Gotenks's hands. When Goku was in the other world, some guy caught him off guard and striked him in the head so hard he returned to his original, evil Saiyan-self, but he managed to hide it from everyone, even Vegeta
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