About Mesirah

“An informer (moser) is also excluded from the mitzvah of brotherhood.”

(Sefer Ahavas Chesed, Chpt. 3, by the Chafetz Chaim ZT”L)

The Chafetz Chaim is stating in regard to mosrim (ie those who deliver innocent Jews to the government) that they are excluded from the community of Yisrael and are considered as non-Jews.