When Lydia interrupted Collins as he was reading Fordyce's sermon, Collins said that she simply could do "no wrong." At that point, Collins was being polite and Lydia's tragedy with her elopement with Wickham had not materialized.
However, afterwards, when Lydia eloped and left her family's dignity in shambles, Mr. Collins reported to Eliza how bad the situation was, and gave his opinion (much contrary to that given in the first instance) that Lydia's own death must have been a better ending than having to shame everyone else.
Hence, the contrast is amazing from doing "no wrong" to wishing her death. :)
Collins arrives at Mr. Bennet's house in Ch.13. In the next chapter after he has had his dinner he is seen in conversation with all the members of the Benett family. Its very obvious that he is doing his best to make a very good impression and win the admiration especially of the Bennet girls, because he has come to Longbourn with the main intention of marrying one of therm.
Soon after dinner and tea the family moves to the drawing room where he tries to impress the Bennet girls by reading aloud to them. To the shock and surprise of both Kitty and Lydia he announces that "he never read novels" and begins to read from Fordyce's "Sermons" - an obviously dull, sentimental and moralistic book - only to be rudely interrupted by Lydia.
However, Collins who is
"much offended, laid aside his book, and said,
``I have often observed how little young ladies are interested by books of a serious stamp, though written solely for their benefit. It amazes me, I confess; -- for certainly, there can be nothing so advantageous to them as instruction. But I will no longer importune my young cousin.''
He is very courteous and polite because he doesn't want to offend the girls by being very moralistic otherwise none of them would agree to marry him.
However, after he has married Charlotte he reveals his true colors. Collins is a hypocrite and he pours out his venom in his letter in Ch.57. He does so in order to avenge his shame and humiliation in being rejected by Elizabeth who is now on the verge of marrying Darcy and by being snubbed by Lydia in Ch. 14:
``I am truly rejoiced that my cousin Lydia's sad business has been so well hushed up, and am only concerned that their living together before the marriage took place should be so generally known. I must not, however, neglect the duties of my station, or refrain from declaring my amazement at hearing that you received the young couple into your house as soon as they were married. It was an encouragement of vice; and had I been the rector of Longbourn, I should very strenuously have opposed it. You ought certainly to forgive them as a Christian, but never to admit them in your sight, or allow their names to be mentioned in your hearing.''