Then perhaps you should have thought it over for more than ten seconds to get rid of all the GLARING PLOT HOLES IN IT.
That's like "of course this equation is right...it took me ten seconds to think of it"

You just said yourself, they had a plan. If Flint Marko just kills some random guy to get his money while the other guy is stealing money and then they take his car...taking the money from the bookie with him. The 10 dollars Flint probably got off of Ben doesn't mean sh**.
Point is, Flint wouldn't kill a guy just so he could run away with less money...makes no sense.