I think Robert Kirkland is Lorelai father .. See his reaction in e5×08 when Lisbon look at a picture and ask Lorelai mother "is this your father ? "  she said " no , its my husband.He works in Munich " She could lying and Kirkland warn her to not tell anybody that he is her husband !!
So , he kill Lennon because he kill his daughter !

Sorry for my English :)