Okay, so here is what I think (If Aang didn't killl Yukone but took his bending away): Yukone is Tarrlock and Amon's dad. Yukone showed Tarrlock the motions for blood-bending (even though he can't bend because Aang took away his bending). Amon learned it and figured out a way to permanantly(?) chi-block a bender (I think it can be undone somehow). Amon and Tarrlock never got along and Tarrlock bullied Amon, which induced Amon's hatred for benders. Amon started the Equalists' movement and Tarrlock found out but, rather than tell everyone that Amon was his brother, decided to take care if him himself. Then Tarrlock talked to Amon and they decided that Tarrlock would gain power and do things that made what Amon said about benders right, and push more non-benders to Amon's cause. Then when all the benders, except Tarrlock, had lost their bending, Tarrlock would rule with Amon, as the only bender left, in dictatorship over the city, and eventually the whole world. Then Amon said that he would agree to it on onecondition: that Tarrlock wouldn't break his promise and become the one and only dictator, with Amon out of the picture.
So, what do you think. I know, a little crazy, but it does make some sense.