Bro you're treating it like its real life shit.
Manga logic works different. If shanks and co didn't do ANYTHING after KOing Kid and killer, and just let them go, who's to say that they won't recuperate and comeback and stir shit up again?
Midd pirates literally were doing that. Benn took Midd's arm off and let him go. Look what they're doing once again?
They're youngsters. shanks literally says so. They're gonna get stronger and stronger. This time, Shanks was there to protect his notoriously weak fleet. But what if he isn't there next time, against a stronger kidd pirates?
It would be because Shanks showed mercy that his allied fleet dies at the hands of Midd.
And since you're equating this shit to irl, take a look.
Look at what the giants are saying. If they point their weapons at them, the giants have the right to self defense. They just invoked that right.