When Steve, being blind, detects a presence in the house, he turns off all the lights (including the lamps). He couldn't know if they were on or not.
Lynn cocks the hammer of her semi-auto pistol and that sound is what temporarily blind Steve shoots at to kill her and save himself. The problem is that because she had just shot at Steve, the hammer of her pistol would have already been cocked (by the slide). If she'd have been using a revolver, the scene would have been more believable and possible, but NOT with a semi-auto pistol..
When near the end, Steve shoots and kills Lyn dead, he appears relieved it's finally over.
Yet how could he have known he had actually hit her?. Considering he was temporarily blind, she could of been standing anywhere in the room with out him possibly knowing where.