In keeping with a viewpoint common at the time, Peter apparently grew up believing that the Law obligated him to love only fellow Jews. It may have seemed completely normal to him to view Gentiles as enemies to be hated.
وَبَدَا لَهُ مِنَ ٱلطَّبِيعِيِّ جِدًّا أَنْ يُبْغِضَ ٱلْأُمَمَ بِصِفَتِهِمْ أَعْدَاءً لَهُ .‏