Basically I'd like to side with the pirates/Furrante (not Aeldys). Can I still kill the slavers?

Feel free to be as spoilerific as you want, I'm past avoiding walkthroughs.

In my playthrough I wiped the slavers and Furrante was more disappointed than mad. He treated it more like business, rather than anything personal, so he was still willing to cooperate with me despite that. I think that all factions will work with you unless you screw them over royally. In the end you will have to choose one faction (or go on your own), you will notice that point for sure.

Ok, so taking and doing all those quests that have me wipe out the slavers is a-ok with Mr. Furrante. Thanks.