^ Excellent points, Nerys
I personally have always been very ashamed to admit that I, or anyone, could have a 'dark side'. I've spent my life trying to hide it, even eliminate it. But as Tuvok once said in a VOY episode (I'm paraphrasing here), "To deny one's dark side is to create an opportunity for it to escape".
Edit: I found the transcript of the scene I was thinking of. It's from "Cold Fire".
Kes: To be honest, I never want to see that part of myself again.
Tuvok: To which part are you referring?
Kes: To the part of me which got pleasure from destroying the plants in the airponics bay. To the part of me that was tempted to go with Tanis. I never realized I had such dark impulses.
Tuvok: Without the darkness, how would we recognize the light? Do not fear your negative thoughts. They are part of you. They are a part of every living being, even Vulcans.
Tuvok: The Vulcan heart was forged out of barbarism and violence. We learned to control it, but it is still part of us. To pretend it does not exist is to create an opportunity for it to escape.
For Garak's part, he did seem ashamed for what he'd done after the drug was purged from his system. And even so, you can't punish people for having feelings (hate crime laws notwithstanding), only for their overt actions. If Garak would have never killed a human without the drug, then I guess that's enough.