Let’s move on to the quote itself: So Simon being kind of creepy (and really not caring about the possibility of being arrested) jumps behind the fence and sneakily watches Luke. Simon does this because Luke tells him a really bad lie about Clary being out of town to visit distant relatives. Don’t worry we are going to talk about Luke’s horrible lying skills later on in this chapter. Right now I just want to talk about how when watching Luke, Simon sees him packing away a glowing sword, and instantly come to the conclusion that it’s a magic sword!
For most people when they see extraordinary things in real life, the they then would automatically try to find a logical explanation for the extraordinary thing. It’s just how the modern human mind works, and for Simon to automatically jump to the conclusion that the sword was magical is really unrealistic. I mean most people in Simon’s situation would probably just think it was a trick of the light, or that it was a fake lightsaber sword; but no Simon instantly thinks it’s magic.
To add on even more stupidity, Clary just decides that all he’s already got undeniable proof that something is going on. And instead of coming up with a believable a lie, she decides to tell him the truth! Clary doesn’t think for one second about the possible consequences that will come as result of telling Simon about the shadowworld.
Clary is essentially bringing this mundane person, with no powers, or no abilities into a world that would probably try to kill him for knowing about the shadowworld. Clary essentially just painted a big target on Simon’s back.
TL;DR This passage is full of negligence and fail, and I’m regretting my life choices right now.