The misconception only comes from Adam Lanza and Elliot Rodger, but sadly that's two killers in two years.
Of course, most people don't make the obvious connection that if autistics are 1 in 45 of the population (including the whole spectrum), and TWO carried out mass shootings, that means that 1 in every 3 million autistics is a mass shooter. Personally, those odds are in our favor.