Luke Smith was downloaded with more than only 21st century human DNA. He knows that Sarah Jane loves him because he is human. That's why she kept him. He knows that there is something wrong with him, something different. So, when he dies and comes back, he runs away.
Three years after Murtagh left the Empire to hide away, Thorn convinces him to start over somewhere new. Eragon helps him start over where he can finally be himself, where he could have a family…in a new world. Harry Potter was a baby very like Murtagh. Raised by those who hate, with the expectations of those in power upon him. Murtagh decides to be his family.