Cats don't meow as a threat. They growl. The varying tones and pitches in which they do so carry information of their own, and sometimes are in similar ranges to a meow. But listen closely to the timbre and intonation next time you find the chance, and you'll probably find yourself able to notice how similar the two sounds are not - not least in that a growl carries much more information.

You're right that meows are by default how cats talk with their kittens. They also bring kittens dead or nearly dead prey, as the first step in teaching them how to hunt. That they choose to use the same idioms with us I think constitutes an eloquent comment on how clever they find us.