It's already been mentioned, but involuntary intoxication is an affirmative defense to any crime regardless of the mental element. Heck, in Canada, iirc, it's a defense to crimes of intent. Especially considering that the Empok Nor drug was designed to induce violence, I think it'd be easy enough for most to forgive Garak.
Now, O'Brien probably harbored some ill will, but then again this isn't the first time he's faced mind-control. Garak's situation on Empok Nor is analogous to that of his own wife in The Assignment. Beyond that, we've got the many times he's been around various types and degrees of mind-control already in his professional life: the Conspiracy parasites, Geordi "My VISOR is a Liability" LaForge, the takeover of Data's body for Starfleet-contrary purposes on at least three occasions...
It's likely that a lot
of Starfleet personnel have either face mind-control techniques directly or have heard about it, so Garak getting hopped up on crazy gas and going on a killing spree is probably par for the course in this service.