Oh, it gets better my friend. Try going through that level when you haven't done the Granny side missions and then neither she nor Slackjaw show up in those sewers. If you're experience is anything like mine, you'll ghost that level, kill absolutely NOBODY and still end up with a Hostiles Killed count (presumably because Slackjaw still dies even though you don't have the option to rescue him).
I think I'm going on a "FFSKILLEVERYONE" playthrough to let off some steam.
Do it. It helps. After spending so much time sneaking around and letting people live, it is a tremendous relief to kill everybody, everywhere, using every possible method at your disposal. Try *everything* when you kill everyone.