I can think of multiple reasons why he might have brought the gun. This is one of them. Shooting the parents is another. He also could have planned a double suicide with her (really, murder /suicide). He may have thought he would need it to "convince" her to leave with him as well, if she was in fact done with him.
After listening to the audio that Wyle posted earlier, it's clear that she called 911 after her parents were shot, but before he killed himself. For me, this means that this wasn't a planned thing that she was in on. But I also wonder if he expected her to just leave with him afterwards, but she was horrified and called for help, and so then he shot himself.
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I don't feel that she planned on her parents being killed but it wouldn't surprise me if she'd planned on leaving with him that morning, nor would it surprise me that it was just their usual ritual, and she'd lied to her parents about breaking it off with him. I can see either scenario b/c she and her brother are alive.
It could very well have been the latter of the above, and he'd scored a gun somewhere, and brought it with him, every time, and everywhere, and no one knew. Honestly, I went in and out of my house, during the night, all the time, had someone cut the engine, and roll up into the drive, and I was off. Never got caught. However, the S-F family had company for Christmas, and the routine was "off" in their home.
The aunt and uncle alerted the parents to a noise, they'd likely not have heard, otherwise. Apparently the aunt and uncle felt it was a significant enough noise to warrant
getting the parents, rather than just peck on the niece's door and ask if she was okay (which is what I'd do if it were my niece and I heard an odd noise in her room). However, if my sister told me that my niece had been having a boy over, against their wishes, in the middle of the night, then, I'd definitely knock on my sister's door, if I heard a noise(s) coming from her daughter's room.
I think he had no intention to kill her, or her brother, because her brother went right on out, and she's alive. If the parents hadn't caught them, he'd likely not have killed them that night. They had a houseful of folks there. You know she told him they had company. I'm leaning toward caught in the act, panicked, shot them, then, already being very troubled, turned the gun on himself.
I listened to the Broadcastify files and mostly heard LE talking. Is there a 911 caller file?