What does George's conversation with Slim reveal about George's past treatment of Lennie in Of Mice and Men?
In this conversation, George reveals that he hasn't always treated Lennie well, but that one near-tragic incident taught him a valuable lesson. Early in their friendship, George explains, he used to take advantage of Lennie and play jokes on him because Lennie was "too dumb even to know he had a joked played on him." Because he trusted George, Lennie would do whatever George told him to do. George found these jokes fun: "Made me seem God damn smart alongside of him."
George continues, however, to tell Slim how and why the joking came to a stop. On one occasion, George told Lennie, in front of a group of other men, to jump in the river. Lennie did so immediately, and almost drowned. George and some others had to pull him out. George finishes his story with this poignant detail:
An' he was so damn nice to me for pullin' him out. Clean forgot I told him to jump in.
After that near-tragedy, George never tricked Lennie again; instead, realizing how much power he had over Lennie, George took care to never again take advantage of him and his trust.
As the above answer clearly explains, George's confidential conversation with Slim reveals that he sometimes used to take advantage of Lennie in the past simply because he could, and it gave him a certain sense of power.
George no longer attempts to take advantage of Lennie in any way, but we can see that he still talks to Lennie quite harshly at times when Lennie's slowness has exasperated him beyond measure. We see this in the opening scenes of the novel, for example, when he lets rip a tirade about how Lennie keeps him tied down all the time. This clearly shows the frustration that George often has with Lennie.
Yet Lennie never once takes offense; his dependence on and affection for George is total. And, as has often been observed, George really needs Lennie as much as Lennie needs him. For all the disparity between them, the two have become essential companions for one another and share the dream of settling down one day on their own bit of land. When Lennie dies at the end of the story, George can no longer entertain such a dream.