@Terry,
I guess it comes down to a persons capacity for free will,self control,ability to reason clearly,having clear perceptions,ability to understand the full consequences of their actions,ability to be in control of their choices without influence from illness, and ability to regulate emotion.
If a person with a coexisting mental illness (or lupus or cancer etc) is able to do the above things and still chooses to do a crime,then its possibly an act of the will and they should take personal responsibility because it is seperate from the illness they may have.
At the same time,there are people whos actions are due to their illness.
What seperates them from people from people who have lupus,cancer,or an broken leg etc is that the organ that is affected is the brain as opposed to a body part.
If the brains personality and behavour area/s are affected,then it can cause that person to act in a way/do things that they otherwise would not have chosen to do (regardless whether the cause is depression,schizophrenia,autism,dementia,CNS Lupus,Encephalitis,tumor or any other pathological process affecting those brain areas).
@Former-Member mentioned one example.Another example might be a father or mother with depression who commit suicide and killl their children too.Often the depression is clouding their thinking and makes them believe the children would not be able to survive without them.
I agree with you that if a person with mental illness is treated aggressively,without respect or dignity,mocked, or treated as if worthless,that it would cause them to feel angry and that's natural.