Since casual players often don't carry sideboards, I'd go with Contagion and Diabolic Edict (they hit nonblack), and which one depends on the situation. Contagion is key in the early game as you can cast it instantly without missing a beat and take out two creatures, especially annoyances like River Boa. It provides extra flexibility for weenie vs weenie matches, and later on, you can use support like Cursed Scroll to round out your anti-creature arsenal. Diabolic Edict would be more useful for more control-oriented decks that need ways to take out larger creatures, especially Morphling. Yes, it is obvious that this sucks against weenie decks or, say, a deck that casts Erhnam Djinns with Llanowar Elves, but if you catch the opponent at the right time, you can get rid of those regenerating, untargetable and protection from black creatures. Maybe one thing that was missed is Massacre, as it kills protection from black weenies as well and is an improved Stench of Decay. And a note about Nekrataal... note that the wording on it is more powerful than the wording on Bone Shredder. Bury vs destroy, right?