Remember what Yoda said, Anakin although was the chosen one, the profecy may have been miss interpreted, just because he is the chosen one he is better than the rest, it just means that the force is stronger on him, and at the end Luke ended up stronger...
Obi Wan was his master with more combat experience, he was a tactical general in the Clone Wars. Thats why he said it was over because he had the higher ground, he knew the advantages and the tactical possition he had, Anakin was driven by hate and made a bad move.