With his "God-like eyes," Slim is kind and keenly perceptive:
His ear heard more than was said to him, and his slow speech had overtones, not of thought, but of understanding beyond thought.
Keeping in mind that it is always essential that any critical analysis of a character be made based upon the evidence of the text, it is, therefore, only fitting and in character--rather than any reader's opinion--that Slim should be the one to speak to George and to comfort him, thus conveying Steinbeck's sympathy with his character George.
Additional support for the appropriateness of Slim's arrival and words to George are also found in the passage in which Candy must allow his dog to be shot. When he looks to Slim "helplessly," he realizes that "Slim's opinion were law"; he looks
a long time at Slim to try to find some reversal. And Slim gave him none. At last Candy said softly and hopelessly, "Awright--take' im."
So, since Slim's words are also "law," the significance of his comforting George and saying, "You hadda, George. I swear you hadda" makes his judgment the final say so, "the law." Thus, the reader is made aware that the other men will make no judgments or remarks to the contrary and George will not be implicated in Lennie's death. This is the greatest kindness bestowed upon George by the man "with God-like eyes," a kindness that reinforces the interpretation of Lennie's death as a mercy killing.
Slim is one of the good guys, and he clearly understands that this is a horrible thing George had to do. If he had ranted and raved and asked George a lot of questions, he'd have made George feel even worse. He may not have understood exactly how this must have felt, but he sure has more compassion than any of the others.
I tend to think Slim and George are rather alike, and if Slim had been in that position, he would have done the same thing. Thus we hear no accusations or recriminations or doubts. Slim knows that, despite it all, George loved Lennie and only did what he thought was best for him. In that case, what else can he do but show compassion.
After George has to kill Lennie, Slim treats him in a very compassionate way. He shows that he understands what George did and he knows why George had to do it.
To me, Slim does this because he is the only really decent character in the whole book (outside of Lennie and George). By contrast, Curley and Carlson do not get it. They do not have any sort of understanding of how George would feel. This shows that Slim is actually a good person. He tries to understand other people and he is not narrowly selfish and vicious the way the others are. If there were more people like Slim around, the Georges and Lennies of the world would not have such a rough time trying to achieve their dreams.