Good on bringing up Hazlan. I want to bring up early stages in Zach's struggle with the drug, part of the story that was never told.
There are perfectly legitimate reasons for Faerun characters to hate it. Hazlik is a Red Wizard. Zach used to be chaotic good, he had a lot of radical left-wing beliefs (There are groups in Faerun with radical left-wing beliefs). Zach used to believe the drug was inherently bad news because Hazlan was involved in it, and slaves were probably used in farming and harvesting it. One day, his good friend Dimitri told him to lighten up, and gave him a dose of it. It sat in his bag for a long time, he didn't want anything to do with it but figured the pain relieving properties might make it useful in case of emergency.
Then one day, all of the other Vardo members disappeared. Dimitri had been gone for a number of days, and Jinx got fed up and walked off mid-shop, leaving all of her inventory on the ground. Zach panicked, he was the last Vardo left. Under the stress, he took it to calm down. There's a post in his Vardo log where he's cataloging Jinx's merchandise while drugged up, a number of the items he just calls by the first thing to come to mind.
So there went his stash. One day, he was at the Vistani vendor, and he saw another dose of it. The mental gymnastics began, he thought to himself, "I'm giving money to the Vistani. Not Hazlan." Once again, it sat in his bag for a while. What triggered him using it was Lofwyr going into the drain and killing every single Vardo and caliban there.
His all-out addiction really began after Mephisto's Darkon plot. It hit Zach so hard, he just didn't care. I suppose you could trace the roots of his fall back to that point, it's when he gave up one of his morals for something.
My point with all of this is describing that if you're from Faerun and have a reason to not like Red Wizards or slavery, you've got a reason to not like opium. For Zach, it was never about what the drug did. It was the politics behind it. His history with the Red Wizards is also why I had him fall to opium specifically, because OOC I love irony.