Was Nightmare ever surprised by just how mortal some mortals were? (Break easy, die quickly, etc, either by accident or time)
I don't think he was surprised by how mortal they were exactly, I mean the first thing he did after corruption was obliterate everyone from their village. He probably saw a fair share of wounding and death when he and Dream were kids and then after the apple incident I imagine he spent a lot of time just wandering aus causing accidents and death so he could suck up that sweet negativity.
I think he was surprised to start caring about it though.
Growing up and being immortal (and also all the bullying and corruption and [gestures vaguely at Nightmare's situation]) probably warps your sense of attachment. Lifetimes pass while you stand still so why would you bother getting attached? Mortals probably seem like flies to him and Dream, they buzz around for their short little lives and then they're gone, he and Dream continue going.
But then he got Killer, on more of a whim than anything. Take in a mortal and task them to fight his brother, either they do well and it's effectively a living shield for him against Dream's arrows, or he forces his brother to kill another monster in self defence and gets to feast on the grief. Win/win for the goopy man.
And Killer did well, so Nightmare kept him. And Killer turned out to be loyal, so much so that it surprised Nightmare a bit. His only real interactions with mortals to this point had been betrayal and fear after all, and Killer didn't seem to project either.
Killer needed company though, and his brother had also recruited some help, so he ended up with Dust (and that worked out fabulously as I'm sure you can imagine) and then he ended up with Horror. And at some point, when one of them tried to get in the way of an arrow fired at him, it had stopped being amusing and started being alarming. It was no longer "this mortal will block my brother's attacks" but "don't jump into attacks, do you have a death wish??".
Suddenly the reality of just how fragile his mortals boys are is something he absolutely dreads having to face











