OK, but...
Z'Phir doesn't know whether the Air Warden is the horror, thinks you are there to talkto him, knows Ro is in the house, and knows the Horror can jump from body to body.

Oh and there are two City Guards at the front gate...

How far are the guards from reinforcements?

seriously? you're going to burn the house and kill the guards?

despite having no IC reason to do so?

Did Ro's thing start getting hot 100 yds from the front gate or 100 yds from the house itself?

Those guards have obeyed him in taking him into custody.

I'm terrible with distances. It was when you approached the house. Almost certainly not the Guards.

Though you had no real option to openly discuss it without giving the game away to Z'Phir.

"Honest, we started that fire because we were fairly sure there was a horror in the house, and we killed the guards in case they rescued it."

"Tell it to the Magistrate..."

You really are bad at predictions.

The Werefrog was planning on getting the guards to help us fight the horror, but they would probably want reinforcements.

well that is a relief...

although, now that you mention it, burning down the house and killing guards would be a more efficient method of dealing with the horror. Whoever survives the house burning down is obviously the horror.

Because people never escape from burning buildings
and collateral damage is always welcomed