Also thinking through who actually killed Cora.
Cora gave Snow the candle initially, and quite possibly brought it into existence; at any rate, she put it on the radar as a possibility. Cora also killed Eva and Johanna, giving Snow the desire to kill her in return.
Henry brought them back to Neal’s apartment, where Hook found them–we’ve already seen that Rumple blames him for this part of it.
Hook used poison he created to attack Rumple, and that injury was transferred to Cora by the candle.
Rumple gave the candle back to Snow and suggested that for Henry’s sake as well as her own desires, she arrange for Cora to take the hit and save his life.
David failed to persuade Snow to give up her desire for revenge before she did the deed. Not much guilt here, but he might feel guilty about it all the same.
Snow lit the candle and manipulated Regina into putting Cora’s heart back, and obviously she feels most directly responsible even before Cora dies, trying to stop Regina.
And poor Regina put that heart back and watched her mother die in her arms, the same mother that she hired Hook to kill years and years ago, from the wound that Hook gave Gold.
It was definitely a convoluted group effort, to say the least.
Keeper of Belle's Gold magic, sand dollar, cloaks, purple FTL outfit, spell scroll, library key, copy of Romeo and Juliet, and cry-muffling pillow, Rumple's doll, overcoat, and strength, and The Timeline. My spreadsheet: https://drive.google.com/file/d/0B6r8CySCCWd9R0RUNm4xR3RhMEU/view?usp=sharing