She had an affair

But not once did Nathan consider the possibility that Ryle was the cause of this mass destruction. The person who had kept him on the edge for so long was the son he looked down on. 

In Nathan's eyes, Ryle was only capable when he inherited the legacy of Kingsleys. Without the name and identity, he was not a threat at all. While he had plans to let Ryle take over his position, Nathan was unbothered that Ryle would become a threat to him after that because he had ensured to keep the real powers in his hand. 

And once Si Yena gave him a child, hopefully a son, it would not be too late to raise the child into the true master. Till then, Ryle could handle the hurdles and make things smoother for his child. 

''Guards! Are you all dead?" He shouted, in part disbelief and part rage as if he could not accept it at this point but not a leaf moved in the huge mansion, ''You son of a bitch! How dare you backstab your own father!"