He was faced with a trespasser in his shower. Nobody is going to believe a guy in the shower was a deadly threat, not as long as he's staying in the shower. Washington law does not grant the authority to use deadly force to expel a trespasser. It also does not create a presumption of a deadly threat because they are in your "castle", as some states do. So the only reason deadly force would have been justified is if the homeowner was facing a deadly threat himself. He clearly wasn't, hence the murder charge.
Who knows what the business owner told the cops, but he could make a case that a casual check told him an intruder was likely on premises, he armed himself after this discovery, and when confronted, the intruder became a threat.
Proximity and ability are there, assuming there is no big physical mismatch in the business owner’s favor. I’d wager the cops heard something that makes the intent component dodgy. Even something as simple and natural as saying “get out our I’ll shoot” when you’re blocking the only exit could cause all kinds of legal problems.