Gary hated Jack. Gary worked as a barman in Jack's pub. Gary also lived in the pub, in the pokey little spare room upstairs. Gary had agreed to the arrangement when he was new in town, and thought it was a good deal. But now Jack kept back money from his wages to pay for room and board, and Gary could never put enough together to move somewhere else. Jack didn't know any of this, and thought he was doing Gary a big favour, and would have been mortified at the suggestion he was causing problems in his life.
And because Gary hated Jack, he stole from him. It started off small, with the odd shot of whisky when he wasn't looking, but he quickly moved onto overcharging and short-changing the punters who were drunk enough, swiping the odd bit of float from the till, never enough to cause worries, just enough to annoy. Gary made sure he always did it when one of the other staff was on duty as well, just to make sure if there was any suspicion it would remain diffuse.
But Jack was a trusting soul, and never suspected a thing.