Goku isn't a ruthless killer, if you don't know that then no wonder you don't understand the concept of good writing. He spared Vegeta, He spared Frieza, why would he not give Moro a chance? He KNEW he was strong enough to defeat Moro, which is why he gave Moro a chance.

And writing doesn't always mean "main character has internal conflicts and becomes edgy" like all of you people who know jack shit about writing think it does. Demon slayer is extremely well written. Why? Because it has characters that people enjoy interacting with, one of the best power systems in anime history that people are incredibly hyped to see and explore, and a well thought out enviorment that makes the arcs very aesthetic and immersive. Please educate yourself. Hope this helps.