You get the point why it is a morality choice but you miss the relevant context.
The morality choice between the glenval bandit and later on going back due to the withering is due to given authority and ability to resist succumbing to impulsive behaviour.
While going to Glenval he was tasked of patrolling the road and given authority to kill bandits on spot by Athagan. Additionally he was not able to bring back the bandit for trial as he still had to go to Glenval and having a prisoner either on carriage or following on foot would be just a disadvantage to everyone involved (the party, Glenval's populace) and this bandit in particular already commited to attacking the party.
Later on in the withering arc the sudden kill of the bandit is not due to given justification of the local lord but by his anger. He has the time and he has the possibility of serving the bandit to the lord for trial. Especially since he flung a fireball first and the bandit just surrender without prior attack but only threatening to attack.
To put it in perspective: A police officer shooting an armed criminal who already started shooting vs a police officer taking custody of a criminal who saw his accomplice being shot after drawing his gun.
Yes, MC has the authority to kill the bandits while doing the patrol mission, but there's big difference in killing an bandit that throws their weapon down and surrenders to killing bandit that is fighting to the last. Even if the bandit originally attacked the MC's party.
To respond in your modern perspective: Even if criminal has shot at the police officers, they still can't execute the criminal if the criminal throws their weapon to the ground and informs that they surrender. In that situation the law requires police to capture the criminal alive, sure in some places around the world the cops might get some leniancy for still killing the criminal, but in the eyes of the law they should arrest them when they clearly have indicated surrender by throwing their weapon on the ground and raising their hands above their heads. If the cops kill a criminal that indicates that they surrender, even if they had killed another cop before hand, then it is revenge murder perpetrated by the cops. And no one should want that, cops murdering people is not a way to deal with crime and punishment, since they aren't the judge nor executioner. Now if the criminal never surrenders, then it's acceptable if they shoot the criminal dead in case they can't disable the criminal, since cops should always try to capture the criminal alive so society can have real justice.