To be fair to Goku, he usually left his villains alive so he could fight them again later. Not exactly morally upstanding, but you gotta respect his inability to really give a shit about casualties. That's what constant access to the dragon balls does to a man, I guess.
Alright he gave second chances after he utterly humiliated them and he's sure he can beat them up afterwards but he aint above murking Freeza, Cell and Buu, he straight up broke Nappa's back just to stop him
but what changed Piccolo was having a son 🤔
Vegeta changed after the 'humbling' that was Freeza wreckin' his shit before bearin' witness to the first super saiyan in that damn carrot
(never watched pervy-ass Dragonball and don't plan to 🤷🏿♂️)