L.M. wrote: "I'm writing an MG fantasy adventure. It's upper MG; the main characters are 13. I was wondering if there are certain dark themes that you can explore with upper MG? Like, death, murder? Or self-sacrifice, even if you lose your life? Are weapons like guns and swords considered okay, or too hardcore (the use or the mention of them?)?"
You've already received some great answers here, but I will say that most of what you list here happens in my MG series (Loki's Wolves, Odin's Ravens, & the 3rd one which isn't out yet; Little Brown Books For Young Readers). The one exception is the gun thing. That particular thing can be tricky to manage. However, we have a murder (by a child), death of another character, monsters, near strangulation, family members consigning a child to be sacrificed, fights with the dead/trolls/a dragon, being runaways, self-sacrifice, and . . . various other "dark" things. We've not met resistance with it. We've had a couple starred reviews, came in as an honour book in the Ontario Library Assoc for our age group, & in general had a great reception.
The only area that any reviewers have critiqued negatively have been "Norse words are hard to pronounce" (which, honestly, is not actually OUR fault) & that it wasn't dark enough! ---> "... young readers may happily turn the pages in little danger of losing sleep. Armstrong and Marr are both pros at spooking kids. Here’s hoping that with the next instalment they really start to show their fangs." (QUILL & QUIRE; April 2013).
So, I don't think that dark is a bad thing overall, esp for older MG.
That said, younger MG can go dark too: Neil Gaiman's GRAVEYARD BOOK & his CORALINE were both very dark. Neil's GB won both Newbery AND the Carnegie. Holly Black's DOLL BONES was too. Neil won a stack of awards for those books, & Holly picked up 4 or 5 starred reviews & a Newbery Honor. Dark can be quite fine
