Wash pulls a face because yuuuck. He is very much a bootshine kind of soldier. He's not exactly a germaphobe but he likes things to be very clean.

"I'll go get some bleach to pour on them, just to be extra sure they're dead."

Crushing them and then pouring something toxic on them might make doubly sure they're dead.

He goes back to the same supply closet that Guts got the gloves from and comes back with a bottle of bleach, to pour on the remains when Alia is done the squishing.