Been playing  RDR,  I completed the   Completed Fort Mercer and Nosalida Hideouts in Single Player an Still didnt get the   Instinto Asesino Trophy yet.  Completed Nosalida Hideout twice and still no trophy for the Single Player.

Replay the Fort Mercer mission. The story mission does not count towards the trophy.

Yeah you will see a text pop-up once you finished the hideout properly. I am playing on PS3 as well and had no problems getting this trophy.