Why was meta-crisis Doctor dangerous?

The meta-crisis Doctor was dangerous because he would go to the extent of genocide, while the current Doctor wouldn't.

Let's try to think of it in another fashion, if he was EXACTLY the same and memories were enough do you think that the Meta Crisis Doctor would have actually committed genocide? What actually happened was that this new Doctor was also half human and therefore he was different in that regard to the original Doctor.

Since the Meta-Crisis Doctor was born in battle, even though he shared all the same memories of the original Doctor he still needed somebody to guide him to be a better person.

Another point is that the Meta Crisis Doctor wasn't necessarily a better person pre-Rose existence, if this was true he would have found a way to defeat Davros without killing the majority of The Daleks. Either way one of the other main reasons The Doctor left Rose with Meta Crisis Doctor was so that he could give Rose a "normal" life, in which they both grow old together.