Goku did it willingly and knew of the consequences, thus ending the last unpredictable and well written saga in all of DBZ. I'm glad Piccolo killed Goku, I finally got to see his stupid ass get murdered, and damn is it satisfying; especially since it's done by my favorite character PICCOLO. Oh and in one of the Dragon Ball Z Tenkaichi's they actually do have that alternate story line. If Raditz became good and all, I don't remember how it all exactly went down, but you can try and look it up if you'd like. I personally don't think it would have mattered if he became good, the story would have been the same either way. Only this way there's one more punching bag for the bad guys, since all everybody ever did was get the crap kicked out of them til Goku came to save the day.