I know this is ancient, but it doesn't matter.
Sasuke is far less-self-righteous and what he believes himself to do is far less terrible than what Ishida thinks he does when he kills a hollow.
He thinks he is erasing their souls, which is the worst thing you could possibly do.
The hollow are the souls of humans who died in despair (except a few who actually did terrible things before they died) and to judge their very souls for such a thing is completely insane and evil.