Some people who never claim responsibility for their actions seem to always place blame on anything or anyone other then themselves. Plus if blaming a violent crime on "having a mental illness" can reduce time in jail, get the person out of jail or just put into an asylum I think they will do what ever they can to try to prove it was due to the illness. The thing is having any mental illness affects everyone differently and it could be some of the reasoning behind some people doing terrible things but that does not mean everyone who has that mental illness will do those terrible things too.