You could just kill the guard by leaving them in the bag
I would immediately shift your alignment to chaotic evil for it because you're literally making them suffocate to death over a period of about ten minutes, but it's still RAW.
Except if the guard manages to rip a hole in the bag from the inside, they can destroy the bag and everything inside it will get scattered across the Astral Plane. Pretty bad for the guard, but better than a slow death being suffocated. Plus you don't need to eat or breathe on the Astral, so they could live there indefinitely.
Not if they have a blade. Sticking anything sharp inside the bag tears it.
Also, I would much rather suffocate over 10 minutes(which would suck horribly) than live out the rest of my natural life floating alone in the astral plane.
And that's only if they recognize what's going on. If you're just minding your business and suddenly it's dark and hard to breathe, there are a bunch of monsters, spells, and magical phenomena that are possible culprits.
You're right, and the rules (both 3.5 and 5) even explicitly state that after ten minutes the creature suffocates. I was hoping that the RAW was simply "holds enough air for a Medium creature to breathe for ten minutes" which I could try to argue was merely a statement about the inner volume of the bag.
I get that it can be painful. it just seems like it would be less painful than an eternity on the lower planes. And I wouldn't want to play a character that does that sort of thing, but that basically means I can't play D&D without homebrewing a less dark cosmology. If you are playing it with the cosmology they give you, and you're not careful to keep your enemies alive, then you're doing stuff worse than this constantly.