The motivation behind why Romeo all of a sudden needed Tybalt dead is on the grounds that he slaughtered his closest companion Mercutio. Romeo needs Tybalt dead as requital for him executing Mercutio. He demonstrates this when he says, "Now, Tybalt, take the "miscreant" back again that late thou gavest me, for Mercutio's spirit is however a little path over our heads, remaining for thine to stay with him. Either thou or I, or both, must run with him"
