Here’s what I suspect. When the owner entered the house the first time, he saw or heard something suspicious. It didn’t rise to the level of calling police, but he didn’t want to investigate further without a gun. Maybe he heard the shower, or saw something out of place. He left, got his gun, and came back to see what was going on. Then he was surprised by the guy in the shower, and panicked.
Yep, that sounds FAR more credible.
Who’s to say the dude wasn’t hiding behind the shower curtain with a gun? Does the homeowner have the obligation to open the curtain to determine if the intruder is armed?