I've been married 38 years.
Would BM have chosen murder? I don't know, really, but I can see him secretly being upset that his poor wife did not die of that cancer -- and make him a sad, grieving widow who can, in a year or two, attract a younger wife. He may be internally angry that this did not happen, and at very little provocation he may have shoved or hit SM and killed her in an outburst.
A mid-life crisis can make some people do just about anything to reclaim their youth (as in, get a young girlfriend).
I don't think the length of a marriage precludes either party from wanting out of a relationship at any point, but most people use divorce! However, there are those people who want the money of the relationship and don't want to share it.
There are also people who are conducting secret relationships that overwhelm their common sense. Go take a look at chumplady.com to read some serious horror stories about what happens when one party wants to get rid of the other party. There are some women posting there that have had attempts made on their lives (eek) and survived. Most are going through knock-down, drag-out divorces in which one party becomes a complete maniac. Most of the partners being chumped had no idea their spouse was so 'unhappy' with their long relationship.