The other Cultivators remained silent while William disposed of their leaders. It wasn't that they were cowards, rather no one would waste away their life just to save face.
In their case, it was already honorable to surrender to William, who had defeated both Supreme Left and Unbreakable Right in battle. No one could call them a coward for lowering their heads to such a person.
William stored the bodies of both Vishnal and Perion, opting to save them for Necromancy later. As for the rest of the Pillars of Justice members, cast enslavement Arrays on any below the Soul Formation Realm while killing off those above that.
He only knew how to create an improved Perfect Enslavement Array and it would need more time to create one at the Complete level. While improved Perfect Arrays could restrain Soul Formation Cultivators greatly, it was still too easy for them to break free.
As such, it was too risky to enslave anyone above the Nascent Soul Realm.