Post crisis Superman kiled the pre crisis level pocket universe phantom zone criminals.
Superheroes don't kill because there's usually other options, like super jails and such. In the case of Dragon Ball, no prison made can contain a lot of those characters, so killing is the only option.
Goku understood that when he fought Piccolo Sr. He killed much of the Red Ribbon Army, too.
He spared Piccolo's son partly because Kami can't exist without him being alive. No Kami, no Dragon Balls.
He helped Piccolo kill Radditz, too.