Witcher 2 makes all of the games mention look like sat morning cartoons, Tactics ogre let us cling together, final fantasy tactics war of the lions and Vagrant Story all have very mature storylines plus they are all written and directed by Yasumi Matsuno. Deus Ex 1 and 3, Fallout 1 and 2, Parasite Eve 1 and 2 are also very dark. Dark souls, Demon Souls are very good. Witcher 1 is also very dark and mature. Radiant Historia has a surprisingly dark storyline given its artstyle and storyline that deals with time travel. System Shock 1 and 2 might also be worth checking out they are alot like deus ex and have very dark storylines.