Here is a hypothetical situation:
A person falls and breaks their left arm. That person is in bad pain so the doctor legally prescribes Percocet for pain. The person takes their prescribed dose of Percocet, gets in the car to drive home and runs over a child on a bicycle. This person could then be arrested for DUI and possibly manslaughter even though the drug was legally prescribed and he had a valid drivers' license.
Now look at this situation. GZ had a license to carry a concealed weapon and he was taking some legally prescribed meds that could have had effects on his thinking or ability to make a good decision. If those drugs caused him to make a poor decision (such as to follow and shoot someone), he can still be in trouble just as the hypothetical person was. After reading the information on the links upthread, GZ could have even thought he was in danger when he wasn't while taking those two meds. IMVHO the fact that he was taking legal medications and how they could affect him is very relevant to what went down that night.
Do you have any examples of a person who was taking legally prescribed medications (and not over dosing on them) being arrested for DUI manslaughter?