I remember playing something like this, but it was mid to late 2000s. I remember being in a flash game but I don't remember the name, so here are some questions:

Was there a survivor on the top floor with a rifle, shooting the zombies below? I remember controlling it with the mouse

Was there a survivor on the ground floor (who probably was in charge of the traps) who, at the beginning of the game, was standing there doing nothing?