I think there is a good tactical reason
If Martin tries to kill Susan, then Susan will not give in easily (because she is fighting for her daughter and is unaware of Martins long con)
On the other hand, Martin sacrificed himself making it easy for Susan to kill him (by giving her extra motivation and by giving up himself)
Also Harry was figuring out that the loophole while chatting with the red king and then having a soulgaze with Martin, I am not sure Susan was onto it until near the end (I think she and Martin went by themselves to rescue Maggie).
Finally there is a more of a surprise for the Red King this way. If Martin killed Susan it would be unexpected because the Red King might have other plans for Martin
From the Red Kings perspective, Martin would kill Maggie and that would take care of Susan and Harry and Martin is still available to continue as a spy.
Also, if I was Martin I would feel guilty like hell for selling out a little girl and a friend no matter the cause and considering that there was a low chance of success. Time do die.
Basically Martin saw the opportunity and jumped on it after triggering the soulgaze with Harry.
This post has been edited by nacht: 21 June 2013 - 09:48 PM