So what do you believe is his involvement? That MH may have discussed a potential motive with him on the night that they were together before Tara's disappearance? I think that's a possibility.
If Tara sent that cryptic e-mail to MH's mother, then I believe Tara knew something about the local LEO's that she wasn't supposed to know. I believe she thought that she and MH could be in (potential) danger from some information that they jointly knew, but in the end, they chose to silence her instead of the both of them.
I have no idea what kind of information that they both could have known, but her e-mail about "something could happen to me, and even Marcus" was leading.