Scary doesn't really equal "mature" to me. In fact the entire genre of "scary movies" is just modern day fairy tales (morality tales about children who don't act morally and the consequences of it).
You cannot possibly have a mature story with as much fantasy as runs around in the DA universe.
Like mature horror - mature fantasy uses the supernatural elements sparingly (think Alien vs. Aliens - the later is not a horror movie).
For DA... that ship sailed when Darkspawn started chatting and demons became next door neighbors - also known as "Every expansion after the initial release of DA:O".
To be fair - there's nothing really dark gritty fantasy about DA:O. The hero is never in peril... he's given a get out of jail free card at the end of the game - none of your companions die and killing them just because you're a colossal douchebag isn't "mature" it really is the opposite. In a mature game the rest of your companions would be flipping their **** the moment you turned on a friend - but the paper dolls just stand there ignoring the fact you're a psychotic murderer.
"Mature" things have consequences... that's never been an element of Bioware RPGs really (or most gaming in general).