You ain’t gonna leave me, are ya, George? I know you ain’t.
No, Lennie. Look down there acrost the river, like you can almost see the place.
Le’s do it now. Le’s get that place now.
Lennie has a breakdown after he imagines a rabbit telling him that George is sick of him. Lennie cries out for George then George reassures him that he won't leave his side.
In order to calm Lennie down, George tells him to look across the river and imagine their future farm with all of the animals that they would have.
George didn't only come to reassure Lennie, he came to end his life in a much more 'peaceful' manor before Curley could get to him. In the midst of Lennie talking about the farm, George pulls the trigger which ultimately kills Lennie instantly.