+ Support For the rules for SCP-076-2 "You may only draw your weapon when you see a humanoid armed with a weapon. You then enter a rage state. While in a rage state you may kill anyone in the immediate vicinity of the armed humanoid that triggered the rage state. This includes personnel who are unarmed. The rage state ends upon all people in your immediate vicinity perishing, or after 5 minutes." You are in the right, you saw Mark with his gun out and had the right to kill everyone in that room.
Also this is not a steamid, a steamid likes look this STEAM_0:0:419545075. (That is your steamid)