Alright, so orc leader is confused because the monsters were given orders, by some unknown being when they were born, to kill humans. However, for him, the appearance of our MC, the monarch, doesn't make sense in this scenario. We can take this two ways:
1. The monarch is supposed to be the one to kill humans and monsters are not to appear when he is alive;
2. The monarch is a being opposed to the monsters and the unknown entity who gave the monsters orders makes sure there's no monarch alive before giving them;