So, to get his fix, he regularly visited a local shop to swipe a pack. And no, the "silly animal doesn't know what stealing means" excuse won't fly here -- Sam clearly showed that he was completely aware of the illegal nature of his deed, treading carefully and waiting for an unobserved moment before striking, as you can see here:
As the shop assistant put it, "He's got it down to a fine art. He waits until there are no customers around and I'm standing behind the till, then he raids the place."
Sam became so popular, and the locals found his thievery so hilarious, that the locals started paying for the chips he stole so that the store wouldn't be tempted to try to stop him. Yeah, way to encourage criminal behavior, people. Now he'll tell all his little seagull friends, and before you know it these damn things will be flocking all over us anytime they think there's a chance they can swipe some food. It'll be like that Alfred Hitchcock movie, only all of the birds will be incredibly fat.
He's an angry bird because all those Snickers bars gave him diabetes.