It seems like the answer is simple after watching the movie - but certain things still get stray me off from a direct answer. People always think they are the exception. Don't deny it. You (whoever is reading this), you think you are special.
I know I think that I won't get caught up in a killing parade and kill my closest ones. But how true is that? Sure, maybe I am not a killer. And sure, I've never even had the desire to kill someone but am I capable of it? Once again, the answer is: sure.
And that's what's really scary about "the Simple Plan".
The fact that that could be anyone.
You don't necessarily have to be a killer to kill.
You don't have to be a stealer to steal.
And worst of all you don't have to be good if you are a good person.
Which is what really made this so-called "simple plan" so ironically complex. The fact that until you are left with the one person you truly trust (yourself) everyone else must go.
Two can keep a secret if one of them is dead.