Don´t get dissapointed, just play Nocturne, the firsta game in the saga and you´ll fill the knowledge void.
The history is really amazing, even if the game don´t have any replay value, but you keep asking for more...
If you need to fill some gaps about the murderer just play Nocturne, the first game oin the saga and you´ll feel more comfortable.
And one last thing, this is really a game that can be rated Mature, and this is the time that the game companies begin to make games for adults.
If you like dark plots involving Templars, order of Scion, and lost artifacts from the temple of Salomon, this is the game for you.