It’s pretty great that treachery is connected to trickery. Apparently, treachery comes from the French word related to deceit and trickery.
Its contemporary usage suggests something much more extreme than trickery. Roads that are treacherous are dangerous – they are deceptive, perhaps but also potentially deadly. I suppose death is the ultimate trick.
And I think Hamlet uses it here because poison is a more deceitful murder weapon than a sword, for example – or a dagger.
It’s not just murder that’s happened – it’s deceitful murder.
We know who did it. And Hamlet probably does, too. But the facts are still obscured, veiled in treachery.