Why was the human Doctor dangerous?

From Doctor Who episode Journey's End transcript:

ROSE: No, but I spent all that time trying to find you. I'm not going back now.

DOCTOR: But you've got to. Because we saved the universe, but at a cost. And the cost is him. He destroyed the Daleks. He committed genocide. He's too dangerous to be left on his own.

NEW DOCTOR: You made me.

DOCTOR: Exactly. You were born in battle, full of blood and anger and revenge. Remind you of someone? That's me, when we first met. And you made me better. Now you can do the same for him.

ROSE: But he's not you.

DOCTOR: He needs you. That's very me.

DONNA: But it's better than that, though. Don't you see what he's trying to give you? Tell her. Go on.

NEW DOCTOR: I look like him and I think like him. Same memories, same thoughts, same everything. Except I've only got one heart.

Maybe, the human Doctor was born in battle, but he had full memories of The Doctor which includes the time he spent with Rose. Why was Rose needed again to make him better? Weren't memories enough? I mean, wasn't he already a better Doctor than his pre-Rose existence?