I refuse to accept mental illness as a justification
You mean as an excuse for behavior?
I don't know why that people justify their behaviors that way
Dealing with people suffering from mental illness can challenging, but keep in mind it's not different than dealing with someone who has physical diseases. It's not like the behavior is intentional.
Mental illness doesn't exist for me. I think the behavior is intentional.
Mental illness is usually called "invisible illnesses" because you can't see it like physical ones, but they are just as valid. How would the people with mental illnesses feel if someone said that what they're feeling is just an act? It's very hurtful, so how about trying to learn about it a little more by having some chats with people that have these illnesses?