Maybe he was just bored and wanted a fight? He's randomly let several people go now.
In this story Demons are like spiders. The moment they're hatched, they're orphaned and the strong survive. They innately learn to craft their web, lure their prey in and eat them without a shred of malice. But they do have intellect and thoughts. Maybe he's saving him for later but doesn't want to lock him up and feed him??
Macht doesn't really make sense as a character, and since he's a demon he can't really have any complex motivations. It seems like he only kills characters if they aren't very important to the story
And why wouldn't he want to do that? I think your theory about demons being like spiders doesn't add up, the don't eat every human they kill, at least he old ones. Unless they loss their appetite but retain their instinct to search and kill for some reason.