Here is my view:
Maybe Sandman aka Flint Marko is just indirectly responsible for Uncle Ben's death...remember before catched he had been kinda boss of the NY underworld.
i.e.
The robber has stolen the money and is looking for a way to escape.
He's nervous and while running out of the building he calls a contact man or sth. else by his mobile phone. But what happens?! - The boss himself appears.
By his 'cool' nature Flint Marko tells the excited and nervous robber to escape by s.o.'s car (maybe he points at Ben who's waiting for Peter but don't want the robber to kill Ben, because of high risk of attention by people,police...).
He hectors uncle Ben to give the robber the car,etc. and suggests him to leave the scenery immeditately. (remeber the Photos on the net which are showing some conversation between Ben and Sandman)
But then Ben said sth. like "I know u, Flint Marko..!" (in kinda slight "pighead manner") -> And while leaving (turend backwards to the scenery:car-Ben- robber) Flint Marko says "Kill him!"
btw. the picture i'm reffering to