It depends. There's a couple ways to end up in the ruthless route. If you want the most painless way: Lie. Tell Asterion you won't ever send him to the valley and then send him during Chapter 11. He will call you out in the most meek and quiet way, but as much of a gut punch as it is, it is nothing compared to what you get sending him out when Argos asks you to command him to come out. It's awful, and because it's something you can eventually make amends for, you still have to send him out again to confirm this is the path you want to go down.
That scene after sending him out with Argos immediately is worth seeing if you want to see Luke in a more nuanced and mature light. He's a "trooper".
ETA re: character - He's justified things to himself. There's one way you can "redeem" yourself in the ruthless route, but it's not an erasure of harm. However, the biggest takeaway without giving too much is that it's not about you. None of Minotaur Hotel has been about you. You are a catalyst/vessel and play a major part of this story, but this is not your story. It's always been about looking outside of yourself to see the humanity in others and that stays true in the ruthless route, as well.