Again, Goku only had "mercy" for Frieza because he knew from his personality, killing is nothing compared to letting him realize he isn't the strongest in the Universe. It had nothing to do with sparing him for any actual moral reasons whatsoever. Fucking hell, if you really realize it, most of the characters Goku has "mercy" for was for completely selfish reasons(Piccolo and Vegeta was explicitly so he can fight them again, Frieza was so he can live knowing that a Saiyan is his superior). Moro was legit the ONLY one Goku has ever spared and it was for a reason so completely OOC, it doesn't even make sense.