Jesus Killer is a slur used to describe Jews in a derogatory way. Christians in the times of WWII when the Jews were most hated used this slur as a reason to hate Jews.
The word Jesus 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 Jesus Killer, it is best to err on the side of caution and avoid using it in any situation.