Well, my 'motive' choice is a little complicated. The short version is resentment, rejection, anger, and vengefulness. The long version below.
The short version of my 'opportunity' is I have no idea, but I believe that to be complicated as well. I haven't worked out a couple glitches in my theory because info is way too scarce regarding everyone's alibi.
Motive. I think MB knew her attacker well. I believe there may have been a significant power-shift in their relationship, based on her fitness passion. This almost certainly boosted her self-confidence as well as her physical strength. She was popular with and admired by her students and other fitness enthusiasts. There have been more than a couple comments made in interviews by family that she was so devoted to her passion that it took up a significant amount of time. One could even interpret a couple of those comments to mean there was some resentment about that. I believe MB had some issues in her marriage and it was stated by family members they were working things out. Evidence indicates despite that, there may have been some outside interests - either still or again.
I'm not going to go into a long list of specifics, but I have pored over interviews, articles, footage, forums, etc., and at every turn I read or hear something that piques my interest regarding my personal choice and a hint at motive. That, plus signs of deception in body language, long-winded propositions/attestations, etc., strengthen my theory. Not to mention physical attribute, which I have covered at exhausting length.
Regarding alibi, I'm just not so sure there. I wish I had access to more data, but I don't think that's going to happen. I'll just repeat myself on this subject: The more complicated and detail-specific an alibi a person lays out, the more interested I get. I don't fully trust eyewitness accounts, as they're not always reliable. I do trust circumstantial evidence such as time-stamped surveillance footage, cell tower pings, recorded mileage, phone data, etc.
