Slim is the "prince of the ranch." He's the consummate Western man: masterful, strong, fair-minded, practical, non-talkative, and exceptionally good at what he does. He is a god among men, and his word on any subject is law. Do you want it in Steinbeck's own words?
…he moved with a majesty only achieved by royalty and master craftsman. He was a jerkline skinner, the prince of the ranch, capable of driving ten, sixteen, even twenty mules with a single line to the leaders. He was capable of killing a fly on the wheeler's butt with a bull whip without touching the mule. There was a gravity in his manner and a quiet so profound that all talk stopped when he spoke, His authority was so great that his word was taken on any subject, be it politics or love. This was Slim, the jerkline skinner. 
Simmer down, Steinbeck. I'm pretty sure the feeling isn't mutual. 
Slim's "authority" means that he gets to decide who deserves justice, and who deserves a little leniency. He's good at reading character, saying "I can tell a mean guy from a mile off" and, unlike some other people in the novel, he's not saying that out of ego. It's just true.
When Slim does lie, he lies for the good of the group or to protect the weak. After Lennie crushes Curley's hand, Slim tells Curley what to do: "I think you got your han' caught in a machine. If you don't tell nobody what happened, we ain't going to. But you jus' tell an' try to get this guy canned and we'll tell ever'body, an' then will you get the laugh".
This is the kind of lie where you tell your friend that you like her new haircut because you don't want to hurt her feelings: the damage has been done, and now you just have to try to smooth it over until the bangs grow back. As unofficial judge and jury of the ranch, Slim gets to decide who's in the right (Lennie) and what the punishment is (not being able to retaliate).
In the end, Slim is the only one who understand what George has done—and why. As the novel's moral center (and possible author avatar), he okays the mercy killing: Never you mind," he says to George: "A guy got to sometimes" . According to Slim's Man Code, if someone has to die, it's better to do it yourself. You can't let a stranger kill your friends.

If that's not love, I don't know what is.