Contrary to popular belief, homocide due to mental illness is declining, at least in England and Wales:

The rate of total homicide and the rate of homicide due to mental disorder rose steadily until the mid-1970s. From then there was a reversal in the rate of homicides attributed to mental disorder, which declined to historically low levels, while other homicides continued to rise.

Paper and press release.

Remember this the next time you read a newspaper article about how scared everyone is because some patients escaped from a mental institution:

We are convinced by the media that people with serious mental illnesses make a significant contribution to murders, and we formulate our approach as a society to tens of thousands of people on the basis of the actions of about 20. Once again, the decisions we make, the attitudes we have, and the prejudices we express are all entirely rational, when analysed in terms of the flawed information we are fed, only half chewed, from the mouths of morons.