It's only when officers start appearing that William feels in the least bit suspicious about Matthew's claim to help him. The officers are followed by Susannah and Martha. Okay, William is an idiot as he actually isn't suspicious about anything even though all the evidence is pointing to the fact that Matthew betrayed him.
William runs towards them and is shocked when their hands are tied behind their backs. Really? Really, William? You didn't guess that the brother to the guy that burns innocent women lied to you? Really? Really, William?