Give me a break, he's the son of a whore and had a father that abused him because he didn't want him. How you expecting this man to be moral upstanding being raised with that type of background.

What you're completely missing is that Betty represents the mother figure he always wanted. Due to that, she can't be the women he truly desires sexually.
Pete was raised rich & privilaged and he's by far a worse person than Don has ever been. Besides, name one single person on the show that hasn't done something morally questionable. Hell, even Don's daughter stole money.
You're also wrong about Don being violent when his back is up against the wall. When confronted by Betty, he didn't attack her. He instead told the whole truth.