On Day 23, the 2nd entrant will give you a silver key (to access a sniper rifle). The sniper rifle works in the same way as the tranquilizer gun, but you only have 2 bullets.
Later on Day 23, while you are inspecting the documents of the 8th entrant, someone will jump the wall and attack the guards. While this is happening, use the silver key to access the sniper rifle, and kill the man in red.
NOTE: killing the man in red will end the game, but you can continue story mode by replaying Day 23 again, and not killing the man in red.