Christ Killer is a slur used to describe Jews in a derogatory way. See: Jesus Killer.
The word Christ Killer is a racial slur that has been used historically to degrade and dehumanize jews. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to jews. If you are unsure about whether or not it is appropriate to use the word Christ Killer, it is best to err on the side of caution and avoid using it in any situation.