Although I would argue against the notion that George's comment has anything to do with fate, the meaning of George's comment here seems fairly clear, at least if we are talking about the meaning on the surface.
George's remark here is made to Candy after they discover the body of Curley's wife. On one level, George seems to be referring to his correct assumption that his friend Lennie killed Curley's wife. George has been through similar situations with Lennie (compare their close call in Weeds mentioned elsewhere in the novel). He knows that Lennie was bound to get into trouble again and now that belief has come true.
On another level, George's statement hints at a similar statement he makes a few lines later about the little house that he and Lennie dreamed of buying:
George said softly, “—I think I knowed from the very first. I think I know’d we’d never do her. He usta like to hear about it so much I got to thinking maybe we would.”
Thus, George's comment also seems to point toward his belief that he and Lennie never would actually get the little house of which they dreamed.