The achievement for Getting Away with Murder is really unlocked when you reach 100%, 100% completion is 50% for main missions, 10% for hobbies and pastimes, 10% for strangers and geeks, 15% for random events, and 15% for others. You just read the archive and the game didn't detect the player completing the last mission, so of course it won't unlock the achievement Mainline missions can be completed in ten hours if you follow the plot.