Hirou didn't bother to verify that the orcs actually were irredeemably evil, which is possible, but you would have to see evidence. Hirou just saw something physically ugly, and his social surroundings expected him to kill them. But the view which he acted-as-if-believed was possible, it simply wasn't true. Arguably my Orthodox Jewish parents committed more blatant mistakes - if far more theoretical mistakes - in endorsing God's murder of the Egyptian firstborn.