Freiza is the one responsible for killing off all of the Saijins because he had a dream that one day one would rise and kill him. He spares Vegeta, Napa, and Raditz and Goku is off on another planet so he also lives.

Freiza is a ruthless being that will stop at nothing to control the universe. During the Freiza Saga, Freiza kills all the Namekians just to get the Dragonballs. When the Dragonballs get used by Gohan and Crillin he becomes in raged and attacks them all. Goku comes to their aid and at the end turns into a Super Saijin and kills Freiza (or they thought). He comes back later in the Trunks Saga but gets killed in one hit along with his dad from Trunks Sword!