Mental illness is not and should not be an excuse for someone to behave badly. It is however, a reason to adjust certain standards of behavior.
You can’t hold a person who is struggling with a mental illness to the same standard of behavior as a healthy person. It’s unfair and you’re both just going to wind up hurting if you do. Bearing that in mind, you’ll need to adjust your standards of behavior to a degree when your in some kind of relationship with a person who has a mental illness. For example, a person suffering from severe depression isn’t going to be able to do household chores like a normal person. They’re lucky if they even get dressed on bad days. So bear that in mind, rather than expect that they keep a spotless house every day, just expect that they shower and get dressed. Some days they might not even be able to do that, but showering and wearing clothes is a much more attainable and fair standard of behavior for them at that point in their lives.