The #1 reason I think they did it is because of everything TM posted after Heather's disappearance.
Between the psycho w**** FB post, saying she had a boyfriend, posting on SM's FB how he loves the Hot Topic fitting room, etc., I'm convinced she was completely out of her mind.
I know some think she's a cool-as-a-cucumber sociopath, but I doubt that someone who's never committed a crime in her life could murder someone and not go bonkers. Everything she posted after HE's disappearance points to a crazed mind attempting to deflect attention and/or use reverse psychology.
And then there were those photos LE showed at the bail hearing of her posing with that big handgun, like she was saying, "anyone who comes between Sidney and me gets shot with this." I mean, c'mon. She may as well have signed a full confession.
There is no way I think an innocent person would do things like this. She displayed nothing but violence and sleaziness since HE went missing. Not one shred of compassion for a family who lost a daughter only 6 years older than her own oldest child.
What gets me is why SM didn't try harder to stop it from happening. Reason with her. "Tammy, if we get caught, we will go to prison. Maybe face the death penalty. Our children will grow up without us." That's what he should have been insisting. It's a great tragedy that he wound up going along with her nutso plans.