Why did Zofia Kossak-Szczucka found a group to help Jews during the Nazi occupation of Poland when she herself was an antisemite? I'm not convinced that it was just to oppose the regime, as there were many other ways to do that.
Zofia Kossak-Szczucka was more of a Polish Catholic nationalist than anti-semite. Her pre-war books underlined the necessity of strong Polish state, and she viewed the Jews as an alien element - she even gained the ire of more hardcore nationalists when she claimed that "baptised Jews are brothers". Her ideas were more related to the general anti-semitism of Catholic Church, which was different from racial-based hierarchy of the Nazis.
However, even the Polish nationalists did not advocate genocide and murder, which was going on in Warsaw ghetto. She did not just fought the Germans, she was a humanitarian - you can dislike Jews for refusing to integrate, but it does not mean that you'd support killing them all. One of the reasons why she disliked Jews was her strong Catholic faith - which was also, probably, why she saved them.
In her famous claim she wrote two important sentences:
"We can't look idly as they're being murdered..." but also "they are still political, economical and ideological enemies of Poland".
Prejudice and hostility do not exclude basic Catholic compassion.