being destructive means that you destroy.
Harmful means that you hurt, or cause pain.
To be destructive, you will probably need to be harmful first.
You can bring harm to a wall by hitting it with a hammer. If you hit it too much, you are being destructive (you are breaking parts of the wall)
You can also be harmful towards people by treating them badly. If you go too far and cause too much harm, you are being destructive.
Hope the helps... happy learning!