Also I think it's worth putting in a couple of sources of Evasion for Karlov like Gift of Orzhova or any other flying enchantment that white has.  If he gets big (and he will) flying could very well end the game for a player.

Palace Seige would be great in this deck.  So would Citadel Seige for that matter.  The seiges are just dope cards lol.

Clearly Athreos is good, but he doesn't gain life so he doesn't really fit the theme.

Any cards with Extort would be great as well.  gain 3 every time you cast a spell for one more mana.

Angelic Accord could also be great to fill your board with big dooders to protect you while you drain everyone.

Thats about all I got!  Had to split it into sections because I was typing a LONG post and it randomly refreshed halfway though >.>

I think Crucible of Worlds can go.

I like pumpable card draw like greed or Erebos better than Underworld Connections.

I don't think you have enough expendable creatures for Westvale Abbey to be effective.

We've already had the Sanguine Bond/Exquisite Blood discussion

Cliffhaven Vampire should be an instant add.

I like Blood Artist more than Zulaport Cutthroat in this deck because it will trigger on opponents creatures.  You don't have a whole lot of expendable creatures in this deck so I feel like you would get more milage out of Blood Artist.