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. :)