If there's a locked door you cannot simply break the lock off. Your guns and melee weapons are rendered useless before this lock. You must first backtrack four levels to Maintenance Room B, get the C Room key. This key will not open your locked door. As you make your way back to the locked door, you will encounter your currently immortal enemy. He will force you out a window or over a ledge or something and you will, magically, end up in a completely different part of the compound. Cue a half-hour of running around, but at least you've got some more ammo! You come to an alter of sorts and use C Room key to open a box which lets you place a sigil in the alter. Upon doing this, you'll hear a faint click. The room you started at is now unlocked. You backtrack more, manage to fight a boss, and get back to that room. This will be the room your ally will betray you in.
And that's why survival horror games are dying out.
And that's why survival horror games are dying out.