Why does everyone assume that Booth and his cohorts were acting for the Southern cause? Is it just the fact that that is the easy explaination for the lazy? What good would the assination of Lincoln and some of his Cabinet members have done the southern cause at that point in time?

There were several other factions in April 1865, within and even outside the Federal Government who had more to gain than did the Confederate States by Lincoln's death. In fact reconstruction may have been markedly different for the southern states had Lincoln lived. Vice President Johnson, Seceratary Stanton, the Catholic Church, even those who were proposing the 13th Amendment would all be suspects to me, given Lincoln previous speeches regarding the slaves where he was at First against freeing the slaves, and then changed because of politics to emancipation of the slaves. But, I do not believe that Lincoln, since he never mentioned this in any of his speeches, was for granting those slaves citizenship and voting rights.

Do we even know what Lincoln's position was regarding the granting of citizenship and voting rights to the slaves? I've never heard that discussed. But you know there had to be discusions at least among the Radical Republicans about the future of the emancipated slaves at the end of the war. Those discussion had to have been about how to use use them to solidify a solid support base for their political party for thier future much like is being done today.