Nathan walked toward the guard, trying to keep his body between that of the two guards in the guardhouse and the guard he was headed to. As Nathan went to accept the flask, he stumbled slightly to close the distance between them (and to cover the pull of the knife into his right hand.
"<Almost time to get to work,>" Nathan said as he reached for the flask. As Nathan grabbed the flask with his left hand, he also grabbed the guard's fingers holding the flask, holding them tight against the flask. In the same instant, Nathan knifed the guard in the chest (hopefully between the fourth and fifth ribs). If that doesn't kill, Nathan will stab him in the neck. Regardless, Nathan will try and steady the guard so that he is still leaning against the post.