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
Even Frieza who is pretty irredeemable came in handy in the TOP. Goku isn't amoral, he's just overconfident he can take down any guy that might cause trouble and gets blinded by his primary motivation to Fight Strong Guys
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 🤷🏿♂️)
Both Piccolo and Vegeta changed after getting a second chance from Goku and understanding how Goku's nobler way of life could bring happiness and even strength to them.
To be fair, they might be referring to the original dragon ball series with kid Goku. It can be difficult to argue that that series wasn't pretty pervy. Dragon Ball Z onwards cut a lot of that stuff out.
Sonic does this too and people complain about it especially in the comics but he doesn't have a strict no kill rule either and it's kind of presented as a bit of a character flaw at times like with Goku
I feel like he might have killed some GUN agents in SA2 though lol
Goku gave them 1 chance to some and if they felt like wasting it kill shot it is. And that was until he grew up and realized he was stronger than most as a kid he decimated the hell of the red ribbon