This trophy is unlocked during the Epilogue, which can only be accessed if you choose to shoot a story-related character at the end of Chapter 15. After you shoot him, the credits will roll, and after about a minute you can press
O, and launch into the Epilogue.
During this short sequence, you will be given to the choice to drop your weapon, or open fire on the soldiers. To gain this trophy, you must drop your weapon. The trophy will unlock shortly after.
To gain the trophy for the other decision, simply reload your checkpoint.
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