explain the differences in mr. collins letter to mr. bennet after lydia eloped to the letter he sent after she was married

Collins' first letter to Mr. Bennet was completely offensive. It's a rant of gossip mongering. Collins even goes so far as to offer Mr. Bennet his condolences...... even worse, he has the temerity to say "The death of your daughter would have been a blessing in comparison of this."

His second letter was one of congratulations for Lydia's upcoming nuptials and a warning against the blossoming relationship between Darcy and Elizabeth. Collins seemed very concerned that his patroness, Lady Catherine wouldn't look kindly on the match.

Both letters are condescending..... and both are insulting. None-the-less, the initial letter was completely offensive, whereas, we can assume the second was written with genuine congratulations and concern.