Creativity is often the result of an unusal perspective. Often those with mental health struggles have a different viewpoint than those considered non-mentally ill.
Van Gogh isn't 'forgiven' for his insanity, it's just a part of who he is. His art gave him a way to contribute to society or to the race as a whole. If he were a double amputee, we wouldn't 'forgive' his lack of legs, would we?
His art, his illness are all a part of what makes him, him.
In the modern era, he would have spent his life in and out of mental health hospitals or jail. He would have been medicated at least once, probably stopping the meds the moment he could to spiral back down into his illness.
Yet I think even in the modern age, he'd still paint. His art would still be spectacular. His mental illness would still a problem.
He's all those things and you can't separate the parts.
He can't help being ill. Therefore there is nothing to forgive. A kid with seizures doesn't ask you to forgive him the seizure.
Only to give any damage he does if he falls on you during one of them. :)