To expand on that, "treachery" emphasizes a connotation of moral judgement that is less explicit in "treason", namely because "treason" is always against the legitimate state (outside of hyperbole), and both the legitimacy of the state and the morality of betraying it are recognized as matters on which opinions may differ. One could be treasonous without being treacherous, or treacherous without being treasonous.

An opinion regarding usage from a native speaker, here: it might for instance be considered treacherous to lure the King into a forest where he may be murdered, in return for a rich reward from a pretender to the throne; but merely supporting the pretender in hopes of a reward, even to the point of secretly betraying the King, might in other circumstances be plainly and distastefully treasonous without evoking the same connotations as treachery.