"Two Black Cadillacs waiting for the right time (to kill him)"
Maybe they were waiting for the right time to......confront him. But as 'luck' would have it, he is accidently killed before they can put their plan in action. The confrontation is averted, but it still leaves them with the fact that they were planning to do
something about his cheating ways. What that something is, we don't know. We know he's dead, but we don't know how. Perhaps the secret they're keeping is the fact that the 2 women discovered his philandering. When they say they
'were'nt going to let him get away with it', well maybe they were planning to tell him the jig was up and that they were both going to leave him. But unfortunately for him he ended up leaving them in a very permanent way. His 'unfaithfullness' is the 'secret' they agreed to keep from everyone and in so doing helped maintain his reputation as a 'good' man. As a side note, the wife sounds generous so I'm almost certain she shared some of her deceased husband's money with the mistress who probably became a good friend.
I think the songwriters deliberately manipulated the lyrics to create the allusion that something more sinister than 'cheating' had happened. But the
'what, how, and
who, is very vague and that along with the instrumentation, and vocals is the lure of the song.