Personally, I'm most interested in the Gargantia crew's reaction to his cold, utter brutality. He probably thinks nothing of it at all, it was clean, completed his objectives, and allowed for minimal effort and minimal casualties to his own allies. Ledo and Chamber will view it as perfect success.

Amy is going to think of it as overkill, and even Bellows is likely to be horrified, considering that she and the other female members of her crew were about to get used as sex slaves and then murdered.

As for how you vaporize someone to sparkly dust? A controlled molecular bond disruption laser, similar to the device from Ender's Game. Maybe one specifically tuned to only break carbon-hydrogen-oxygen bonds, so it stops from chain reacting with the atmosphere. Or, since it blew up the boats but vaporized the people, it is a dynamically-tuned Convenience Beam to keep unnecessary gore out of the series.