Mrs:susan grimshaw and agent milton

The question is / When agent Andrew Milton came to the gang's hideout to arrest Abigail, why did Mrs. Susan not shoot him ?? My theory is that she was busy preparing to leave and Abigail was arrested from the other exit from the cave when she took advantage Dutch get out and stole the key. tilly noticed situation When she was with Jack , a bigail complete her plan of theft, preparing to leave, and cutting off the allegiance of Dutch.so mrs susan not see agent milton and tilly go away without her knowed.