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
Goku's villains (not counting anything past Z cause bleh):
Piccolo: Spared for Kami's sake and Dragon Balls. Totally killed Piccolo senior.
Vegeta: A foe like no other.
Buu: Reincarnation.
Freeza: For me, I'd hesitate to ignore anyone begging for help, especially at his state. "Killed" later anyway.
I'm not sure where to wedge this into the conversation, but Goku also has firsthand knowledge of the afterlife. I feel like that definitely changes what it means to kill someone.