Why does Slim say, "You hadda, George. I swear you hadda"?
Slim is consoling George after George has shot Lennie. It seems that only Slim understands why George has done what he has in taking his friend's life. George has had to make a hard choice, and he has chosen to sacrifice the dream by saving Lennie from a cruel death at the hands of Curley and the mob. Because Lennie has killed Curley's wife--even though unintentionally--Curley is set on avenging her death as cruelly as possible, by shooting Lennie in the stomach so that his death will be slow and painful.
George, on the other hand, takes Lennie's life as painlessly as possible, by shooting him in the back of his "head, at the place where the spine and skull were joined." George encourages Lennie to talk about their dream of the farm and the rabbits so that Lennie is happy and distracted when George pulls the trigger. George has to be the one to take Lennie's life in much the same way that Candy lamented that he should have been the one to shoot his dog. Slim is the only one of the ranch hands on the scene who comprehends George's actions.