That's the entire point.
I'm looking for a protagonist who will do anything as long as it's allowed, but is still classified as a 'good guy'.
Using Black Star as the example, he was allowed to kill the witch and the swordsman worth 100 souls. He would have upgraded Tsubaki into a Death Scythe (despite the name, they're not all scythe shaped).
He was ALLOWED to do it, therefore in the name of opportunism, he should have, for the following two reasons:
1. At that point he had obtained a grand total of zero souls in his entire lifetime.
2. This was a quick, entirely acceptable route to a huge increase in combat strength, and likely a chance he would never have gotten again.
He turned it down because he couldn't kill a witch with the form of a young human female. He may have made the correct moral choice, sure, but looking at things seriously, it was fucking stupid.