Sir Ralph the Rover apparently wanted to put the passing ships into trouble, not the Abbot himself, I think. But yes, he was jealous of the Abbot of Aberbrothok’s fame and glory that he achieved by tying the bell to the Inchcape rock. That is why he wanted to cut the bell and make an end to the Abbot’s growing popularity. The purpose of looting the ships in danger was another reason behind this though.