Why does Alfred ruin Tom's book?

Alfred pours ink on Tom's spelling book because he is jealous. Alfred realizes that Becky does not like him, and that she is only using him to get back at Tom. Thus..... Alfred gets back at Becky through Tom.

Then Alfred went musing into the deserted schoolhouse. He was humiliated and angry. He easily guessed his way to the truth—the girl had simply made a convenience of him to vent her spite upon Tom Sawyer. He was far from hating Tom the less when this thought occurred to him. He wished there was some way to get that boy into trouble without much risk to himself. Tom’s spelling-book fell under his eye. Here was his opportunity. He gratefully opened to the lesson for the afternoon and poured ink upon the page.