Other source:

It looks like the battle is over, as Frieza seems to be defeated and the Ginyu Force has been beaten off King Kai’s planet to Hell by the remaining Z-Fighters. As everyone celebrates, Frieza reappears and spares no time attacking; first he fires a blast at Goku, but Piccolo hurls himself in front of it and takes the blast, knocking him out. Then Frieza fires another blast and uses it to blow up Krillin, killing him. Fueled by rage on the loss of his best friend, Goku’s power increases exponentially, and he ascends to become a Super Saiyan.