I've always wondered that too. Why bother killing your partner? Surely even a messy divorce that leaves you penniless is going to be better than spending years in jail, and leaving your kids with no parents? It is senseless, isn't it?
Nothing is achieved in the end , just an awful lot of misery. Divorce would be the better option in all cases.
Their ego fools them. They think they won't get caught because they're so much smarter. That ego is also their downfall in they are so convinced of their own intellect and are so cocky that they overlook the little things and the obvious is not so obvious to them.