Garak was under the influence of drugs which he did not choose to take, therefore no, he is not guilty of murder in the Empok Nor cases.
On the other hand, if he had chosen to take them
be guilty of murder.
Although of course the ironic thing is, being guilty of murder is something that Garak certainly
is in many
other cases, most of which we don't even know about, but many we do (ie: Tain's enemies, the dude in ITPM, Weyoun etc. etc.).