Actually, although I feel you have some type of vendetta leading me to think you may personally know someone involved, you might want to do your homework.
There are the strange circumstances surrounding the death of dad's wife. There has been a lot of speculation by the locals, probably because they were talking divorce. Some of the pieces don't add up, but he was not investigated. Even a local LE said that we don't know the whole story. So we don't know that he was "respectable". When he pulled me over for speeding, he was very professional and personable, but that doesn't tell me what he is like in his personal relationships.
Then, Megan and the boyfriend were secretly seeing each other, even though he had another girlfriend. Megan's mom threatened bf and his dad with legal action if they didn't stay apart. They were caught together in Megan's bedroom. Keep in mind that if he was charged and judged guilty, he would be a registered sex offender for the rest of his life. If the rumors are true (and who knows if they are - no one knows) and Megan was pregnant, it wouldn't have been too difficult to prove the case. Having that charge on his record would completely change his future - there are jobs he wouldn't be allowed to take, he would have to report every change of locale to the authorities, plus, there are those people (like me) who would not want to be involved in any way in his life - not as an employer, coworker, or neighbor. His dad would have been very aware of the consequences of a charge of sex with a minor.
Megan was very naive - bf could have told her anything and she would have accepted it. He knew this. He took advantage of her - in every way - IMO, right up until the very end.
I agree that mom's behavior is odd, but then, she lost her only daughter. I don't know that she is innocent, but I just don't think she murdered her daughter. None of us know how we would react in this situation until it happens to us, so I feel like I need to give her a break. But we are aware of bf's reaction which included a thorough washing of his truck the morning after Megan's disappearance, and the fact that he never cooperated in trying to locate Megan.