In one of the stories it said Bauer heard them enter his bldg, got his gun and sat in his chair until they started down the stairs. AND according to stories, when his first weapon jammed and the girl laughed at him, he retrieved and used another. He wasn’t exactly ‘trapped’ and unable to retrieve another weapon.
From a legal perspective, what’s important is that they weren’t exactly dead after the initial shots. After his legitimate self-defense, Bauer chose to execute them, thus violating the law
He didn't murder them. They committed suicide using his bullets.