I'm still on the fence regarding premeditation (at least from a legal standpoint - I agree that this doesn't appear to be well planned, but am unsure if it was a crime of passion or if it was something else).
My wife would say he was never a bad guy growing up. He wasn't perfect - was a bit arrogant and cocky, but we all knew many guys (especially athletes, IMO) who were that way in HS. He wasn't a troublemaker. Certainly time and life experiences can change a person, but I think it's accurate to say that nobody who knew him growing up would have ever imagined a scenario like this with BM a POI in something like this.
My wife has said that, literally, SM may be the absolute sweetest person she has ever known. Truly an extraordinarily kind soul. (We joke about how every victim on Dateline could "light up a room" - but the positive comments you've heard about SM are not exaggerations). DW's impression through the years was they were happy together (and not simply posing for FB). If BM is indeed guilty, something significant changed - with him, with the relationship - or both. JMO, MOO.
Just by his body language, I think BM has probably been a bit of a hothead since he was a very young child. However, I also think he's probably come about it honestly by the way he was raised: the baby, only boy, and mom, and two older sisters that doted and indulged him. Seems he's always behaved like the little prince and expects to be treated as such.
I think I'm in the minority here because although I believe money and greed are the catalysts of BM's anger (and ultimately SM's death), I don't see a family in financial straits. Not at all. BM has been working at his craft for more than 30 years. He clearly likes what he does. We can't sleuth BM but just looking over SM's joint ownership since marriage, they've done quite well for themselves.
Personally, I've always believed that BM brings home the bacon but SM is the financial steward of this family. SM is the reason they have what they have!
I believe SM came back stronger from cancer. I think she's come to realize--probably from the new generation she raised (M & M) that BM's narcissistic rage is not normal and other people don't live walking on eggshells all the time.
I think BM has hidden behind the most decent woman ever known to many. She's been his true cover for his entire adult life. We all have that friend that we tolerate because we love the other half.
I think part of this Indiana wedding planning may have included SM's desire to extend her stay and look after her dad. COVID19 was presenting an opportunity to take her minor daughter with her given virtual classrooms. I also think the timing was right for SM-- their former residence previously sold with the final/ balloon payment due in weeks. Maybe it had been discussed before and suddenly BM saw the plan going into fruition.
I'm also not seeing any period where BM/SM have ever been apart. Maybe this would be somewhat of a trial separation for them. I think even the idea of such a thing would put BM into a blind rage. He knows that there'd be more than a good chance that SM would find she's happier on her own than with him.
More importantly, I truly believe just the idea (real or perceived) of BM having to divide his kingdom could cause him to go unhinged. I do think BM killed SM in a fit of rage. I don't think BM would kill over losing SM but I believe he'd kill over money and greed. I don't think it helped that they were alone together -- the daughters away out of state.
MOO
