Massacre cleans up the early part of Izzet Delver's board: non-delirious Dragon's Rage Channelers and the 1/1 Monk tokens from Cori-Steel Cutter all die to -2/-2. It does not answer Murktide Regent, which is at least a 3/3 and grows with each instant or sorcery exiled with it, or a delirious Dragon's Rage Channeler, which is a 3/3. We also have to pay full cost, since they never control a Plains. Surgical Extraction names a single card and exiles every copy of it from their graveyard, hand, and library, so it needs a copy in their graveyard to target. Once a Lightning Bolt, Dragon's Rage Channeler, or Murktide Regent is there, it can strip the remaining copies of that card and blunt their clock. Unmask and Collective Brutality come out because their discard is less reliable against a deck that deploys cheap spells quickly and can protect its hand with Force of Will and Daze. This keeps our proactive reanimation plan the same while adding a sweeper for their small creatures and tokens.