How does Juliet's father's attitude change against Paris and his marriage to Juliet after the death of Tybalt ?
Juliet's father becomes much more hurried about getting Juliet and Paris married, once Tybalt is killed and everything seems to be going badly for the family. Initially Lord Capulet told Paris that he must win the love of Juliet before she would marry him - he seemed almost jovial and benevolent, wishing to allow a bit more time for Juliet to mature and come to love Paris. But when push came to shove and Lord Capulet told Juliet she would have to marry Paris, he was extremely angry with her for saying she would not do it.
It wasn't a situation where Lord Capulet didn't want Paris to marry Juliet. Rather, he was insistent that it be done quickly, and was then furious when Juliet didn't leap for joy at the news.
There are a few different reasons for this change. One interpretation is that Capulet is concerned about his daughter's sadness, assuming that it is because the death of her cousin. He feels that a wedding would cheer her up.
A deeper interpretation, however, suggests that Capulet is concerned for himself and his family. In Act I, the prince threatened Capulet and Montague about severe punishments should more fighting take place. Although Tybalt was killed, he was obviously responsible for fighting in the first place. As Paris is a cousin to the prince, Capulet assumes that a marriage between the prince's family and his own will protect the Capulet's from retribution.