The one thing that really can't be explained...
Or at least, one thing that can ONLY be explained as "necessary for the plot".
When Felix shoots Vera, SHE DOESN'T MAKE A SOUND. No scream, no cry, no "Felix! It's me! Help!"
WHY?
She clearly doesn't die or even lose consciousness - she's plainly trying to open the door while Felix scrambles for the key so he can lock it.
It can't be accidental, to keep silent when in such pain and terror would require an almost superhuman effort. Oh well.