Agreed that we need to listen in; based upon their current impressions of the situation, we might want to let them go or might want to kill them. If they can become convinced that they have the cult problem under control up until our fleet's shadow takes the planet, that would be ideal.
That said, if they plan to call in backup, or to report that a major demon summoning is underway, they'll have to be silenced.
More broadly, I think we've got a couple objectives here. One, to bring down the hive's power and unleash as much tyranid hell as possible on them, using rippers and whatever else we can to tear things us. Two, to support the genestealer cult in whatever plans it has going, allowing them to act as a more standard rebellion and seize weaponry, communications, and the like so that the PDF units and governmental apparatus can't make use of them.
We have some solid plans in support of the first objective, but the second will require actually finding the cult and discovering what they're doing.