Mental ill or evil? I believe that given the right set of circumstances, anyone of us is capable of behaving in a way which we would not have thought possible. We can't say for sure how we would act if such and such an event happened, or if we were caught in a particular situation. We can suppose and hope for action commensurate with our character, but who knows? People both overestimate and underestimate themselves frequently.
Some people who do bad things are clearly mentally unwell, while others are evidently bad people. We all have the potential for good and evil. How many evil deeds are required for a person to be deemed evil? Do acts of charity and selflessness wipe out the stain of what Christians call 'sin'? Who decides who is a good person and who is a bad person? Who has enough knowledge to make such a judgement?
We have all fallen short of the Glory of God.