Dahak, in the thread about the birth of Anakin, suggested that the midichlorians knew about the imbalance Palpatine was about to bring in the Force and created Anakin to restore the balance.
I think that at the beginning of episode 1, the force is already seriously imbalanced. It's leaning heavily to the light side. There are dozens of light jedis, who wield an enormous political power to boot, while the one or two sith have to stay in hiding. Therefore, Anakin is supposed to become evil, because that's how he will fulfill the prophecy of bringing balance to the Force.