Candy was counting on sharing a real home with George and Lennie. He had invested his entire savings of three hundred dollars in buying the house and land. He comes to the barn to talk to Lennie because he keeps getting new ideas for improving the property and needs to talk to somebody about them. When he first sees Curley's wife lying there he thinks she is asleep.
"You oughten to sleep out here," he said disapprovingly; and then he was beside her and--"Oh, Jesus Christ!"
Candy blames the girl for coming to the barn and presumably flirting with Lennie, getting him sexually aroused, and then getting killed when he attempted to rape her. That is the scenario that all the men visualize, including George. They know Lennie was responsible because everybody else was playing horseshoes, and they know that Lennie is always in the barn playing with his puppy. They all assume it was an accidental murder in connection with an attempted rape. This is not really too far from the truth.
Candy doesn't blame Curley's wife for getting killed. He blames her for being flirtatious and promiscuous. This is really not true of her. She is only acting that way because she wants to become a movie star and she is trying out her charms on the men at the ranch. She is imitating women she has seen in the movies, including, no doubt, Jean Harlow. She doesn't really want to have sex with anyone, including her own husband. Lennie did not kill her in attempting to rape her but in attempting to keep her from screaming--although he might have tried to rape her if she hadn't started screaming.
So Candy blames Curley's wife unfairly. She was really just being friendly. She wasn't even trying to be flirtatious with Lennie, but she has been acting so flirtatiously on so many occasions that it is natural for Candy to assume this is what led to her death. He realizes that her death has ruined his chances of sharing a farm with George. Lennie has apparently fled the scene. He will either be killed by Curley and the other men, or else he will be captured and probably executed for murder--or else locked up in an institution for the criminally insane for the rest of his life. Without Lennie the farm dream is out of the question. Candy can't expect George to do all the hard labor while he contributes nothing but a little housekeeping. Besides, George and Lennie were partners; it was their dream. Candy can't hope to have the kind of relationship with George that George had with Lennie.
When George leaves and Candy is alone with Curley's wife, he vents all his anger, frustration, self-pity, and despair on the dead girl.
"You God damn tramp," he said viciously. "You done it, di'n't you? I s'pose you're glad. Ever'body knowed you'd mess things up. You wasn't no good. You ain't no good now, you lousy tart," He sniveled, and his voice shook. "I could of hoed in the garden and washed dishes for them guys." He paused, and then went on in a singsong. And he repeated the old words: "If they was a circus or a baseball game . . . we would of went to her . . . jus' said 'ta hell with work,' an' went to her. Never ast nobody's say-so. An' they'd of been a pig and chickens . . . an' in the winter . . . the little fat stove . . . an' the rain comin' . . . an' us jus' sittin' there." His eyes blinded with tears and he turned and went weakly out of he barn, and he rubbed his bristly whiskers with his wrist stump.