I'm going to go a little bit further for those who don't quite understand.
Without spoiling too much, the reason Dual Destinies has an M rating is because of situation in the last case of the game that touches on the subject of "matricide." For those of you unaware, matricide is the term of killing one's mother. It is for this reason and a few disturbing images that Dual Destinies is rated M.
Eh... Even after playing it, I never really agreed with it being M.
Yes the cutscenes make things a little more 'real' then previous games, but it's really not much.
And Phoenix Wright has touched on matricide
and patricide before.
*Spoilers*
If Athena had been declared guilty of killing her mother, then
maybe I could see the correlation to the M rating.
In Phoenix Wright: Ace Attorney (The first one), Edgeworth confronts his past regarding the DL-6 incident.
He believes that he is responsible for killing his father through most of it, and he is further pushed toward believing it by the testimony of Yani Yogi.
Now if you want to push the idea that it's
matricide and that patricide alone isn't capable of giving an M rating, (Both aren't, really), then we can look to Bridge to the Turnabout, in Trials and Tribulations. Maya is accused late in the case of murdering her mother, Misty Fey, although under the control of someone else. It is proved the guilty one is Godot, though he attempts to cover his crime not by
framing Maya, but rather by trying to do the opposite, make it look like she had nothing to do with it.