While I definitely agree that letting abductors go is bizarre, I do understand Arito's position. He has been followed extensively by others numerous times before and their intentions weren't exactly nefarious so his decision to not slaughter them but capture them is fairly normal. Once they were captured and defenseless, they learned of their true intentions, but is acting like a judge and jury and just slaughtering them in cold blood the right move? Moreover, Arito is no hero; he does not have any responsibility for these people's past and future actions.
The best solution would be to bring them in. However, this is not practical. If Arito has to travel with a large group of bound men he would be significantly slowed down in his travel, he would also have to feed them for the duration of the travel and then he would have to explain himself to the authorities. We don't even know what justice is like in this world. What proof is there that this merchant has abducted before and that he was trying to abduct Arito? Moreover, as a merchant who deals in this kind of shady business, it would hardly be surprising if he had some contacts with the authorities and could find a way out of such situations by greasing the right palms. Unless there's a bounty on Hecknayr, which we know there isn't since he was going around as a merchant at parties, he's just unlikely to be arrested.
So in summary, I understand why Arito did not murder them in cold blood and why he let them go instead of turning them in himself. I don't think it's smart to carelessly judge others and murder them and it is too much trouble to deal with them himself.
With that said, Arito was informed that slavery is illegal and Mia (Miliana) did ask him (in a manner that suggests nothing good will occur to anyone he reports) to inform the Emerald Sword party if he ever met slavers. So I would say that letting them go and sending a letter to Mia indicating that Hecknayr the merchant tried to abduct him will send the Emerald Sword party in an investigation and they might even be able to get a list of abducted children and free them. It's unclear if he has sent a letter and the conversation with Lina seems to suggest that the slavers are being forgiven (but then Lina is also seemingly not taking any actions herself and not suggesting a course of action to deal with these slavers so clearly Arito isn't the only one who isn't taking any actions here).
@Toykio
His mother was an elf, and he's a half, so his father is clearly human and cannot be Orst.
It's possible that the elf tribe his mother is from is somehow related to Orst but it could also simply be that elves admire high elves and in particular a powerful elf such as Orst who is essentially a legend for them.
@crypted
Unfortunately, people often have double standards and are not logical but emotional. Hecknayr is clearly a twisted person with an inferiority complex. I don't think it's bad writing to have characters who have inconsistent beliefs because that's about as real as it gets; people are stupid and blind to the paradoxes in their own beliefs.