In Chapter 1, Steinbeck devotes an entire long paragraph, taking up a whole page, to George berating Lennie. George begins by complaining that his life would be easy if he were not saddled with Lennie. Then he says specifically:
"An' whatta I got. I got you! You can't keep a job and you lose me ever' job I get. Jus' keep me shovin' all over the country all the time. An' that ain't the worst. You get in trouble. You do bad things and I got to get you out." His voice rose nearly to a shout. "You crazy son-of-a-bitch. You keep me in hot water all the time."
Then George brings up the subject of the girl in Weed as the most recent example of the trouble Lennie causes him. This incident is extremely important, since George describes it in further detail to Slim in Chapter 3.
In the first chapter George warns Lennie what to do if he gets in trouble at the ranch where they are going to be working. He tells him to come back to their present campsite and hide in the bushes. That is exactly what Lennie does when he kills Curley's wife in the barn. The reader should be expecting trouble, just as George is dreading.
Heretofore, Lennie has been fond of petting little animals, includiig mice, puppies and rabbits. But the Weed incident seems ominous. Lennie appears to be taking an interest in girls. George does not really know what happened in Weed--a town at least four hundred miles away from where the novella begins. They both had to run for their lives, so all George knows is what Lennie tells him, which is that he wanted to feel the fabric of the girl's dress. But Lennie lies to George consistently. He shows a strong interest in Curley's wife the first time she appears. The interest in petting little animals is probably a sublimated interest in sex. And if Lennie kills all the little animals he handles, there is a good possibility that he might kill a girl.
When George sees the body of Curley's wife in the barn, he says to Candy--and to himself:
"I should of knew....I guess maybe way back in my head I did."
What George "should of knew" was that Lennie was taking an interest in young girls and was too retarded to understand his own feelings or to control his impulses. He is a lot like Benjy in William Faulkner's The Sound and the Fury, an idiot who tried to rape a little girl and was castrated to prevent him from horrors.
George did not witness this incident in the barn, either. He assumes, as does everybody else, that Lennie killed the girl while attempting to rape her. This is not true--but it is close to the truth, because Lennie might have proceeded to rape Curley's wife if he hadn't killed her to keep her from struggling and screaming. He might have raped the girl in Weed if he hadn't assaulted her on a busy street in broad daylight with George beating him over the head with a fence picket to make him let go of her dress.
When Curley's sexy and provocative wife first appears in the second chapter:
Lennie watched her, fascinated.
After she leaves the bunkhouse, Lennie says:
"Gosh, she was purty.
George reacts with alarm.
"Listen to me, you crazy baatard," he said fiercely. "Don't you even take a look at that bitch. I don't care what she says and what she does. I seen 'em poison before, but I never seen no piece of jail bait worse than her. You leave her be."
Here Steinbeck makes it clear that Lennie is interested in soft, pretty things other than mice, puppies and rabbits. In fact, he is potentially a serial rapist and killer.
Trouble is foreshawdowed through Lennie's strength and curoisty. Lennie accidently kills a mouse, and to accidently kill something like that requires a lot of strength. Also, you learn about the woman in the red dress, which is why George and Lennie go to work in their new ranch.
“What’d he do in Weed?” Slim asked calmly.
“You wouldn’ tell? . . . . No, ‘course you wouldn’.”
“What’d he do in Weed?” Slim asked again.
“Well, he seen this girl in a red dress. Dumb bastard like he is, he wants to touch ever’thing he likes. Just wants to feel it. So he reaches out to feel this red dress an’ the girl lets out a squawk, and that gets Lennie all mixed up, and he holds on ‘cause that’s the only thing he can think to do. Well, this girl squawks and squawks. I was jus’ a little bit off, and I heard all the yellin’, so I comes running, an’ by that time Lennie’s so scared all he can think to do is jus’ hold on. I socked him over the head with a fence picket to make him let go. He was so scairt he couldn’t let go of that dress. And he’s so God damn strong, you know.”
It shows there is going to be trouble when he touches the lady in the red dress and she screams but he did know what he was doing wrong, showing he has a disability and will cause trouble without knowing he is doing anything wrong