I didn't think about Septimus III, but yes, I think he didn't like to "lower" himself to a level of someone who kills defenceless people, even Klingons. He wouldn't want to become what he despises. And Klesh was no match for two warships and
the station after the other Klingon ships were gone. Brenok didn't like 100% casualties. Winning the battle would be enough, but wiping out every single person was too much. He won't lose any sleep over this--they were warriors after all--but he didn't like it.