@Meo I said 1 and 15, not all the stuff in between, but I meant 1 and 17, which I knew cause I looked it up but miss typed, thought 15 looked better, something or the other. The biggest point being was that he didn't want to fake his death and if she could kill/do everything on her own, the Demon King would be dead and the Hero would never need her allies, so why would the world make her allies to fight/travel with.
But the fact is that he wasn't one of the "Heroes allies", whom were granted blessings to be "useful" to the hero. What he did do was all the things outside of the actual fighting, which a lot of people take for granted. If I understand this chapter right and the first one, Red/Gideon actually hasn't leveled up in a while, so he became a hindrance when it came to pushing deeper and deeper in to the demons territory.
He was there for what we know as emotional support, but would he know that? His sister never tells him anything (Cause that blessing is f'n dumb), we know she wanted to go after him, but he didn't since again, she never tells him anything anymore. Last time was when they met Lit for the first time, but that was years prior to ch 1, so I can see how he was persuaded to leave. I agree that leaving his sister without saying anything is a dick move, but gaining the status of "The one who left the Hero Party" if this was found out would possibly forced him to become a hermit, since his face would be circulated around the world like a traitor who couldn't fight for everyone's survival.
TL;DR Yes he's a dick for leaving without saying anything.