I’m not looking down in this person, but I’m a little confused about mental illness.

I have two family member whom have or had mental health issues at least once in their life span.

I notice one of them is un medicated and seems to be functioning , she’s stalkerish , has anger issues enough to murder someone. Has tried to kill people before. I notice the still works on a job take care of a child etc have a normal life. I’m wondering how is this possible? Maybe she’s not mental but just has problems ?

Because the other family member has schizophrenia and cannot work and is nearly disabled and is almost bed confined.