What's your interpretation of what Goku was feeling here? I took it to mean he's distraught he was forced to kill Frieza, not because he didn't want to take a life, but because now he doesn't think he'll ever get to fight someone as strong as or stronger than him again. Goku thought he killed him here and believed he was the strongest in the universe, so now he must be like "oh fuck, I'm never gonna get to fight a stronger guy ever again"