Fans of the book k**w that Euron's crew is made up of men he's spared During previous pirate raids. In order to stop them from speaking to each other to rally against him or talk back to him, he cuts out all of their tongues. So when he has his own men do this to the crew from Yara and Theon's ship, he's doing so to replace the men on his side that he's obviously losing in the battle.
Yeah. We've seen some f*cked up characters on Game of Thrones before, but Euron Might be the most terrifying yet (just his entrance to that battle says it all, to be honest).