Asterion, while a good natured person, is still deeply broken from so many years of torture at the hands of various 'Argos'. I think his desire to be free of the labyrinth coupled with his possible hatred towards Argos may have been the reason he left him behind. I think that Asterion would likely come to regret this choice, but since we don't see anything from his perspective after this, we can't know for sure.
It's heartbreaking to think of Argos trapped there alone, just as it was when Asterion was left abandoned, especially after we learn more in the main story about Argos trying to help Asterion. Admittedly he went about it in a somewhat backwards way, but he was genuinely trying to help in his own broken way.
The biggest recurring theme in minohotel is that all of the characters seem to be a bit broken, whether by abuse (Asterion), imposed traditions and expectations ('Argos'), childhood trauma (Oscar), family troubles (Pedro) etc... I won't go through and list every character's trauma, but they are all broken in some way when they arrive at the hotel. Including the MC presumably.
So perhaps this scene is a bleak example of what can happen if that brokenness isn't addressed, and it's left to fester. You continue that cycle of brokenness (*cough* recursion *cough*) and it will keep going until you or someone (multiple people?) else works to heal that brokenness.
Or, perhaps, I'm totally missing the point...