This is what I think.
It was supposed to be a heist. Cash. Art. Jewelry.
Things got out of hand. Change of plan.
Hope he/they rot. Sorry. I do.
I have a son Phillip's age. They even resemble each other. I can't look at his photos. It just breaks my heart.
I have a similar theory, only the original aim was to steal the Mosler. But only SS had the key, so he had to come home. I can't imagine anyone thought they were going to drive away into the sunset with a car like that, but the people who committed this crime don't live in the same reality as the rest of us.