But it's not like the dude even drawsHe goes for the gun because he believes that if people CAN make a difference, they should.
Ben tries to get this across to Peter multiple times, and also tries to teach him that getting petty revenge is...petty.
Peter doesn't GET that until Ben dies, that's the whole point. It doesn't matter if it was a sleazy wrestling manager or a store clerk, neither one deserves to be gotten back at. But Peter's young and still gets off on petty vendettas like getting back at Flash. Which has no immediate consequences. And then when he gets the jerk cashier back, THAT time had consequences.
A hero doesn't get revenge, he does what is right.
the gun in a threatening manner. He basically trips, it falls out of his pocket and Ben needlessly struggles with him.
Edit: To further sum it up, in SP1 I felt like "Wow, Peter really fucked up here" rather than "Uncle Ben you idiot, just keep your head down!" in ASM.