Villain
The word villain, in a general sense, does not apply to the Doctor. But in a way, some people may attach it to the clone. He destroys a whole race in Journey’s End. So, does that make him a villain? Well, according to this article, Russell Davies had bigger plans for the human/timelord Doctor, but because of budget constraints, it did not happen.
Could the Tenth Doctor clone been real bad and done terrible things? He did enjoy destroying the Daleks! The Doctor tells Rose she prevents him from becoming him. So, what if he stays in our universe. What does he do? He is the Doctor, which means he has the same enemies as the full Timelord Doctor. Does he go after all the old enemies he faced over the years?
Davies describes what he has in mind while shooting the episode. He talks about a blue suit Doctor whizzing back to the Shadow Proclamation, wiping them into a war, then leading a Judoon lead fleet to the Medusa Cascade. Okay, that is a lot to digest and a little beyond what fans can see the Doctor doing.
Say this takes place, how does it change Doctor Who? Will it mean the Doctor has a new enemy to fight? Will he be able to overcome his clone? There are many questions to be asked. Does the Timelord intelligence rule his body, or does the human side control him? Fans don’t know what to believe. They do know he destroyed the Daleks in the episode. And the Doctor saw what he could do and sent him and Rose back into the alternate universe.
It may be cool to have a half-human Doctor, but is good for the universe. Everyone knows he will raise his ugly head at least once every season. Soe fans probably couldn’t get enough of seeing him. He would as good of an enemy as the Master, or maybe better. Who knows how things would be if Davies got what he wanted. You can’t say the ratings will be worse, or they may be better. Fans will never know because the budget not being enough causes Davies to trash the idea.