Goku has been sparing his enemies ever since he trained with Kami. In Z, he was mocked by Raditz, Vegeta, Ginyu, and Frieza for not being ruthless. Sparing Vegeta was the only example of him admittedly being selfish. In the case of Frieza, Goku thinks the tyrant being traumatized and living with the shame of being beaten by a low class warrior was punishment enough.
Cell is the only exception in that he's the only enemy Goku actually tried to kill, and needed to bargain with to prevent him from taking lives. Cell Saga is where Goku started being jokey and relaxed because he realized training the mind make Super Saiyan much more powerful than training the body.