From my perspective, when I read the story, I saw it as Candy's tragic realization of what this would do to the dream he had with Lennie and George of buying the house and land they had planned to. They had sent the money, and were working for the rest, but without Lennie or George to help, there was no chance it could ever happen.
Surely, we can't discount the fact that Candy was looking down at a woman who had been murdered, and that something bad was going to happen to his friends because of it, and that all of it was out of his control, but I think the loss of his dream--his Hail Mary pass, as it were, his last chance at security before he died--being lost was what caused him to weep over the body of Curley's Wife.
Crooks actually says it best:
I seen hunderds of men come by on the road an' on the ranches, with their bindles on their back an' that same damn thing in their heads. Hunderds of them. They come, an' they quit an' go on; an' every damn one of 'em's got a little piece of land in his head. An' never a God damn one of 'em ever gets it. Just like heaven. Everybody wants a little piece of lan'. I read plenty of books out here. Nobody never gets to heaven, and nobody gets no land. It's just in their head. They're all the time talkin' about it, but it's jus' in their head.