Ron (as he will reveal next chapter) is panicking. He thinks that he's losing his friendship with Harry, even as Harry thinks he could lose the same thing. So he resorts to desperate measures and makes everything worse. (He would not appreciate the irony if someone attempted to explain this).

He's mad because it's Malfoy, because things are changing, and because he's jealous. Once again, Harry has something that he doesn't. Ron feels that this is the last straw.

Ginny was trying to help. She just really, really didn't.