Ed Harris' lead performance as Walker (who is quite definitely insane) is magnetic, and Peter Boyle delivers an excellent supporting role as Cornelius Vanderbilt in two memorable scenes.
How did the movie treat Walker's restoration of slavery in Nicaragua?
One of the fundamental principles of conservative thinking about slavery and the Civil War and modern-day politics that slavery was a doomed institution, that there was no such thing as the Slave Power and that Lincoln et al. needlessly centralized power. There really isn't much reason I know of to think that Walker was insane but wasn't a reasonable example of the southern way of politics at work. His restoration of slavery so far as I know was a perfectly natural continuation of the domestic policy of using state power in defense of slave property. Turning Walker into a lone madman lets the slavers off the hook, doesn't it?