Depending on how you watch the series, the twist of Leo's killer will be revealed at different moments. Though we never see the killer's face, his identity is revealed by the shirt he wears. As Leo's killer approaches him, the camera pans in on his black shirt with a colorful logo on the front — resembling a kaleidoscope.
In the episode, "White," Roger's son Brad is seen wearing the exact same shirt as he sits with his father in their apartment, right before the heist took place. It's insinuated that after Roger was arrested and sent to jail, Brad wanted revenge for Leo tearing up his family — similar to how Leo sought revenge on Roger for breaking up his family.
In the end, the entire show comes full circle as each character meets a bitter end as a result of their greed and thirst for revenge