Some people may ask now where she's got the gun from. Find the answer here! And yes, the gun was hidden in her handbag all the while.

About her red eyes: It was Death who caused it. The red eyes as well as the iridescent light that surrounds her both symbolize Mercedes' incapability of apprehending her surroundings. Neither was she aware of what actually happened in the cellar (the outbreak of the fire), nor that she had been injured as well.
After the red of her eyes has faded away, the bruises on her face and body and the singes on her dress become visible. And THIS is the only reason why she doesn't shoot her unfaithful boyfriend - she notices the scratches on her hands and wonders where they come from.