There is not enough evidence to conclude that Lennie killed the girl in Weed, but we do know that the incident frightened George. He is aware of what Lennie is capable of, even if he has not witnessed it firsthand.
George does not say what Lennie did in Weed. He simply says, “bad things like you done in Weed.” We do know the incident was serious enough that they not only had to run, they were chased.
"Run us out, hell," said George disgustedly. "We run. They was lookin' for us, but they didn't catch us." (ch 1)
However, later on we are given more detail on exactly why they have to run. Lennie wanted to feel her dress, and she thought he was grabbing for her.
She jerks back and you hold on like it was a mouse. She yells and we got to hide in a irrigation ditch all day with guys lookin' for us, and we got to sneak out in the dark and get outa the country. (ch 1)
George would have reacted differently if Lennie had killed before. He protected Lennie, but when Lennie killed Curley’s wife he acted. He shot Lennie before he could hurt anyone else, and before Lennie could get caught.
It is true that the color red is symbolic. However, in this case George feels guilty because he should have seen the signs. He did not act when Lennie scared the girl. He did not realize how dangerous Lennie was. He blames himself.