The more I learn the more I think he's lying. First, who locks the bathroom door? Closing the door maybe but locking? 4 months is long enough to be comfy with your SO. I think there was a problem for her to lock the door.
As others have mentioned. If I thought that there was an intruder. I would alert my SO and find out where he is first make sure that we're both safe then address the issue. Not shoot first.
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