[Dfesdefu]

What lies behind the backstabbing of Urusander? What's the secret agenda of Renarr?

[nacht]

This question is not directly answered by the Author. It is possible the author will answer this later.
Meanwhile we can only guess.
For one, Renarr is not following a traditional role of dutiful and chaste daughter etc., and is actually rebelling against that stereotype.
She is generally considered to be powerless and harmless and maybe this is a rebellious.

Another option is that she is compassionate and might have to the conclusion that Ursander is dangerous and will cause more damage.

[HoosierDaddy]

A victim of violence, Renarr finds the war ridiculous and terrible. And, despite his words Urusander glides along allowing the atrocities of those under his command, as if he has no say. She finally has had enough of his irony and slides a blade between his ribs.

Done and done.