We shouldn't hate the monarchy, necessarily. We should hate monarchy as a concept.

It's archaic, it formalises and legitimises unbelievable levels of inequality and elitism, and it gives rise to at least the strong possibility (and in the UK's case at least, the actuality) of a tiered legal system, with some laws simply not applying to some people because of their position.

It's a repulsive idea, based on historical might and hereditary right, and with no regard for democracy or equality of all people.