What's the difference between a "mental disorder" and a bad personality trait? Most people are held accountable for their actions. If diagnosed with a "mental disorder" however, they are often forgiven/treated more leniently and exempted from punishment. 

The same thing couldn't be said about a bad personality trait however. No one is exempted from murder for say "not being very compassionate" or "being an asshole".

Mental disorders however only seem to be a term of describe a set of bad personality traits that are displayed concurrently though. So why the special treatment?

What's the difference between a mental disorder and a bad personality trait?
I don't want to address the issue of defining mental disorders in general here, because just about any malfunctioning system in the brain can be considered a disorder. However, some disorders affect the set of processes in the brain which ultimately determine whether or not someone takes an action (the neural basis of volition, or "free will", if you will...). If these decision processes are broken, people shouldn't be held responsible for what they do, because they really have no control. So, a "mental disorder" which exempts one from responsibility can be considered one which consists of damage to the following processes:



Of course, there is the philosphical objection that mechanistic explanation (deterministic or random, doesn't matter) means nobody has free will - but the issue in society is really about free will as the ability to learn rules and follow them. A bad personality trait usually doesn't affect the above processes, and therefore doesn't impede someone's ability to make their own decisions.