So, you see that Leland transforming into "BOB" when committing his acts of evil is a metaphor for Leland's state of denial / insanity whilst he is behaving that way.
Leland's behaviour is a metaphor in OUR world.I said there were many fathers who rape their daughters,and there is no BOB inside them.BOB can be really a representation of the evil that men do,but only here,in our world.These fathers are really insane.And you are right about that they were usually sexually abused as they were children.
But, please understand me the right way,BOB really exists in the world of Twin Peaks,this is some kind of a science fiction or kind of,something like X-files,this is a story about supernatural forces.And it was BOB who made Leland do all the bad stuff he did.You can't deny the existance of BOB.This way the show "Twin Peaks" will become completely different.
Actually I didn't mean it quite like that. I meant that Lynch and Frost tried to say that in our real world those who rape their children aren't always insane persons. They might be lovable persons who just can't help it when they are feeling the temptation. They have a problem grown too big.
Bob isn't just an inhabiting spirit. In the world of Twin Peaks people do have the power to ignore Bob but they are too weak to do it most often. Just like us in the real world. Almost every single day we give in to some kind of temptation (it can be sexual or it can be about food or anything). Bob is just as strong spirit in Twin Peaks as the temptative force is in our world. It's damn strong.
Bob can be both a metaphor and an inhabiting spirit in the world of Twin Peaks. To those people who can't see him, he is the evil that men do and to those people who can see him, he is the evil that men do but with a face so that some people could recognize the evil energy when they see it and that way it could be possible to try to crush the evil force under the good force again.