You are instructed to move an ability from one color to another. This ability must be something used in every set (i.e. discard, direct damage, card drawing etc.). You may not choose an ability that has already been color shifted by R&D. What ability do you shift and to what color do you shift it? Explain why you would make that shift.
Currently, mass creature destruction is a primary ability in white, and secondary in black. Blacks mass kill effects tend to be limited or conditional--Hex, Consume the Meek, Infest, Barter in Blood, and so on--while white gets the big, unconditional sweepers like Day of Judgement. I would shift whites slice of mass removal into black.
There are strong mechanical and creative reasons why black should have access to mass creature kill. For one, black is the color of death, so it shouldnt be surprising for it to kill indiscriminately--thats what should be expected from a significant burst of black mana. Black is also the color most willing to sacrifice its own creatures, and the color best-positioned to take advantage of a well-stocked graveyard through reanimation spells. For further synergy, blacks ability to attack the opponents hand allows it to cripple the foes ability to recover afterwards.
All of blacks biggest core mechanics and characteristics lend themselves well to mass creature destruction; white, on the other hand, is more conflicted. One of whites most iconic strategies is the weenie rush, and thats not usually the sort of strategy that wants to nuke the board. Day of Judgement as a white spell creates tension, forcing white in two different directions as its Ajani Goldmane and Honor of the Pure demand lots of creatures to pump while its sweepers want as few creatures as possible to maximize value. Flavor-wise, while black views its creatures as expendable, white (although it understands that some losses are acceptable) is much more likely to want to protect them.
Logically, this should leave black with simple mass destruction such as Damnation, with white restricted to spells such as Planar Cleansing that, although perhaps more thorough, require a larger mana investment.