Unless Lincoln losses a son or Mary Todd, I think he would only punish those immediately responsible for any assassination attempt.
The best POD in my opinion would be for Davis to agree with Jackson in his call for taking no prisoners and put as many Union cities to the torch as a possible. If say Cincinnati, Evansville and other cities are put to the torch very early in the war and hundreds of Union prisoners are executed after the Confederate victory at Bull Run than the end will be bad for the Confederacy.
As for a long drawn out guerrilla war, I don't see it happening. An astute Union politician, like Lincoln, will make every effort to separate the poor whites from the plantation aristocracy. More likely we'll see a much larger Confederate exodus to Mexico, Brazil and other Latin American locales. Like WWII Germany the Confederacy (even in OTL) was pushed to collapse. In this scenario it will be worse and the war wary population will have the shame of the massacres to deal with as well. Reconstruction will be a simpler affair and there will be no Lost Cause ideology in the US. Instead there will be angry emigre populations of Confederates throughout Latin America, and this will cause a lot of butterflies down the road.
Benjamin