Duncan Idaho said:
Since they were being ritually bleed and using the blood to mark the Sardaukar, I dont like the idea of failed Sardaukar.
Prisoners makes more sense but the idea I like (because of how absolutely brutal and cultish it is), is that they are sacrificed Sardaukar like Lief in the first season of Vikings.
Book spoilers:
I thought the whole planet was the emperor's prison planet. The Sardukar were just the ones who survived through it