Quote:

he was reaching for the syringe of glowing orange liquid that was the oxygenating potion, pushing it into Hermione's neck at what Harry hoped was the carotid artery, to keep her brain alive even if her lungs or heart stopped, so long as her brain stayed intact everything else could be fixed, it had to be possible for magic to fix it, it had to be possible for magic to fix it, it had to be possible for magic to fix it, and Harry pushed the plunger of the syringe all the way down, creating a faint glow beneath the pale skin of her neck. Harry then pushed down on her chest, where her heart should be, hard compressions that he hoped was moving the oxygenated blood around to where it could reach her brain, even if her heart might have stopped beating, he hadn't actually thought to check her pulse.

The paragraph gives two reasons (Harry might not have found the carotid artery, he might not have done chest compressions correctly) why the potion could have failed to work. I don't see a compelling reason to hypothesize that there was some other force behind Hermione's death here.

Was she murdered by someone who was more concerned with killing her than making sure it looked like an accident?

If this were true, why bother with the troll?