I have a now-estranged family member who is extremely charming when he thinks it will serve his purposes.  Behind closed doors with his family he was a manipulative, violent, perfectly dreadful individual.  
He is either criminally insane (although not formally charged or convicted of any crime), or he is evil - depending on one's perspective.
Personally, I lean toward the 'evil' verdict.  
But those who suffer from legitimate mental illness (i.e. schizophrenia) are certainly not evil and (unfortunately) are statistically far more likely to be the victim of violence than to commit an act of same.