'What did you do to my brother?'

No. It's just showing what influence Eric had on Rey and desensitizing him to killing. Eric felt nothing for people in an every man for himself apocalypse and was all about his recapture of his dog's remains who he loved. I thought that made a fascinating ending and the only attachment you feel is for animals.

Rey killed the guards who had captured Guy Pearce's character at the military base. But I don't think he had ever killed anyone before that (since he was kind of surprised by how easy it was). I don't think they had ever trusted Rey to kill anyone as part of his brother's gang. They treated him as someone too dumb to ever make a decision on his own and he never questioned the way he was treated until Guy Pearce showed him that he could think for himself and take charge of his life.

Rey eventually realized that his brother had treated him badly his whole life and didn't really have his best interests at heart. He had abandoned him for dead without even bothering to recover his body. He just drove off and left Rey there, wounded and presumed dead. When Rey pointed the gun at his brother, it came as a big shock to the brother. He thought his brother was a brainless, spineless dolt who would never have the guts to threaten him or even question his authority.