Are mentally ill people more violent? A lot of people are afraid of people with mental illnesses because they think they're more violent but is that true? 

I think it's wrong even though I don't really understand the pages I found on google. I found this article from University of Washington which states among other things that "the vast majority of people with mental illness are not violent". I understand this article but it is really short.
'Mental Illness' covers such a broad range of illnesses that it would be very difficult to answer your question. However, general consensus is  that: people who are receiving an effective treatment for a mental illness are no more violent or dangerous than neurotypical/ 'healthy' people. People with a mental illness are more likely to harm themselves or to be harmed. There is a weak association between violence and mental illness, especially when alcohol or drug misuse is involved. Early treatment of mental illness is the key to preventing violence. This article covers the topic fairly thoroughly:

Link, B. G., &amp; Stueve, A. (1995). Evidence bearing on mental illness as a possible cause of violent behavior. Epidemiologic Reviews, 17(1), 172-181. 

This is from their conclusion on mental illness and public safety: 


  People with mental illness pose no greater threat to public safety than do such other "high risk" groups as men or young adults or people with less than a high school education.