I’ve heard it said many times that a Jew that’s killed because he’s a Jew is considered as if he died Al Kiddush Hashem. Where is the source for this?
While the source for this is unclear, it is an idea which has become widely accepted among believing Jews. One who is threatened with conversion or giving up his life, and gives up his life has died “al kiddush Hashem”. Many who are killed for being Jews would not have been killed had they converted to another religion. Presumably these kedoshim would not have chosen conversion had they been given the chance and would have opted to die “al kiddush Hashem”. In fact we know stories of many non believing Jews who refused to convert even when it cost them their lives. Perhaps based on this or a similar line of reasoning the above idea developed.