I get that he wants to defend the kids but the guy just give them a single panel to answer... they could barely even react when he made the questions... one didn't even finish his thought. If actually wanted to get information and not just kill them, unless he can read minds, then that was a very dumb and rushed decision...
That Clank sound effect, probably signifies them taking a combat stance. Gray doesn't wear metal armor, so it's unlikely to be from him. He also already knows they're hostile, "we'll report to the bishop and finish it within a day". They came at night in the dark, and they left a probably skilled combatant/kidnapper hiding behind their home.
If they had no ill intentions, they could have lowered their swords/relaxed/said something to the effect that they're not here for criminal reasons. But he knows he's caught them saying something that he shouldn't hear. His cackles are already up, from when they were so evasive at his home. Now they've revealed themselves to be working for a pretty high level official of the church. So, why wouldn't they reveal that it's a church thing instead of pretending to be mercenaries? Shenanigans.
And he's not really after information? He wants to solve the problem. Information would be a bonus, but he already knows the hero is in the forest, that purported mercenaries were openly asking about Ariamel along with the hidden arrival of the hero, and that Ariamel has been in the slums for quite a while and no one has asked about her until now.
Edit: And he knows he's in a JRPG game. Of course the Bishop is a villain.