I think the Sligs having mindless conversation would be good. They can have conversation without humanising them, as long as they appear either very stupid or rotten to the core. Make it reflect their sadistic nature, and have them tell crappy jokes and stuff. With the right dialogue they'll seem even more inhuman than they did before. The Super-Mutants in Fallout 3 had some mindless chatter which made them a little more believable, without generating much sympathy for them, so it can work.
Also, I sort of agree with Sekto - somewhat scripted animal deaths could add to their brutality. Maybe not to that extent, but certainly having several different ways of being mauled by a slog after being taken down could be quite entertaining. Think when you're killed by Necromorphs in Dead Space.
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