You and Alex were fighting on different sides, during the battle Alex makes a mistake and goes into enemy ground which is where you are, hiding.

When you see Alex you lunge towards him with a knife, plunging it into his shoulder as he yelled in pain, you raised the knife to make a finishing kill…

But what you didn’t know was that Alex had a son, he was behind you, just when you were stab Alex, his son shot you in the back