Rotation looks like this:
Plague Strike, Icy Touch, Pestilence, Blood boil, Death Strike, Horn of Winter, Death Coil
Death Strike, Pestilence, Blood Boil, Death Strike, Death Coil, Death Coil
After the first 2 sets of runes continue to spam Pestilence, Blood Boil, Death Strike & Death Coil.
Later on you can use 2x Blood Boil at the 2nd set of runes and refresh diseases with Blood Tap + Pestilence when they are about to run out.
Yes unholy is sweet for AoE while you don't sacrifice anything on main target. And even greater if you focus on AoE.
I think using 3/3 Wandering plague, 5/5 impurity, 5/5 Desolation and Glyph of Disease would improve single target as well as AoE damage greatly.