With all due respect in return, I don't know the exact situation with your client so that is why I provided caveats - and no you can't accurately predict who will go into a violent rage - depressed or not. Perhaps nobody should get a driver's license or CCW because anybody could go into a violent rage at any time? Me thinks not.
Those suffering from mental illness, assuming they are not violent, have rights. To assume that anyone who has been diagnosed with dysthymia or major depression, when they have never otherwise shown any behavior that indicates anything violent or suicidal, to assume they will go into a violent rage just because they are depressed is close minded and a violation of that persons rights.
If I were denied a driver's license based on a standard diagnosis of dysthymia (assuming mild to moderate symptoms and no history of suicidal ideations) I'd expect a strong advocate in my lawyer who would fight for my rights and sue the pants of the DMV and state. Then, some lawyers might prefer to make it into a long, drawn out fight in order to pad those billable hours. Other lawyers might be misguided and not understand that those diagnosed with dysthymia are in the majority of cases not dangers to society or themselves, but due to this ignorance those lawyers might not be strong advocates for the rights of their clients.
As for your client, no, I do not have all the facts, but do please tell me how I don't know what I am talking about. If your client is a suicidal person who suffers from road rage then no, of course he should not have a driver's license. If, however, your client just has standard symptoms of dysthymia, such as fatigue, a decreased appetite, trouble sleeping and some indecisiveness, what does that have to do with the potential for going into a rage and why should that person not be able to drive a car? How many non-depressed people get behind the wheel to go to work after a bad nights sleep while missing breakfast? Should we yank their licenses?
To further your statement that I don't know what I am talking about, I indicated that the inability to sleep well due to depression and impaired sexual relations can constitute a disability under the ADA. Please tell me how I don't know what I am talking about per the below links. Towards the bottom of the first link we read that the inability to sleep well due to depression can be an impairment per the ADA (as quoted by the EEOC) and in the second link the U.S. Supreme Court has ruled impaired sexual relations can constitute a disability under the ADA. Note I say "can" constitute a disability because circuits have different rulings on sleep as an impairment, but agreement is across the board at the SCOTUS level on sexual relations - however an impairment is always judged on a case-by-case basis. So, if your depression causes a major life impairment, you are covered under the ADA - however, if that impairment were to affect your ability to drive or own a gun in a safe manner, all bets are off. No doubt it is not a cut and dried legal area, but the mentally ill and people who suffer from depression do have rights and they deserve strong advocates - especially so deranged individuals like Cho can hopefully be found out and properly treated before we have another VA Tech. At the same time, those suffering from depression don't need to be pre-judged as suicidal, murderous maniacs just because they are depressed. Just as tens of millions of firearms owners did not kill anyone yesterday, 20 million or so depressed people in the U.S. did not either.
http://www.eeoc.gov/foia/letters/2003/ada_definition_sleep_apnea.html
http://www.ncd.gov/newsroom/publications/pdf/limitation.pdf