Mission 16: Stopping Marchenko
[MISSABLE TROPHY / ACHIEVEMENT]
For this you must pick up the kill switch (story item) before going to the Marchenko Boss Fight.

On your way to Marchenko you'll get a call from Vega and Mission 17 will start. Now instead of doing mission 16 select mission 17 in your mission list and follow the waypoints there.

A door that was previously locked will now be open (alternatively, you can also take out the bald enemy near the door to get the keycard, the dude at 0:55 in the video).

In the room to the left of where you find Miller is a box under a table. It contains the kill switch. Now that you have the kill switch it's time to go to Marchenko (back to mission 16). During the opening dialogue you can use the switch to kill him instantly.

Note: Doing this will void the Pacifist trophy (counts as a kill).

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