No, you guys are seeming to get lightside and darkside mixed up. Allow me to clear this up. The Force doesn't have a darkside and a lightside. First off lets just explain the force in a way like this; The force is like a pool. Most pools have a shallow end and a deep end right? Will, the sith like to use emoution and pure force power. But sinse the old Jedi where not allowed to use the force to cause harm, and this deep end of the poo of pure force can be used more for harm maybe. So the Jedi claimed that part of the force to be "dark" and evil and such. The deep end is more force then anything it uses more of the force and such. The Jedis are on the shallow end of the force pool. So I will end it there, sinse if I explain it in detil it would be very long to explain.
I think the real question is "Was Anakin's fall to the dark side a failure?"
Anakin's destiny was to "bring balance to the force." When he fell to the dark side of the Force, it was thought that he wasn't fulfilling his destiny. However, as we all know, in RotJ he was the one to kill the Emperor. Would he have been able to do that as Anakin Skywalker? I don't think so. The only way Darth Sidious was going to be destroyed was by a betrayal from his right-hand man Darth Vader.