Who said "I've killed, and that will haunt me forever."?

This quote is attributed to Arrow.

I've killed, and that will haunt me forever. - Arrow

"I've killed, and that will haunt me forever."

Arrow Death

"My name is Oliver Queen. After five years in hell, I have come home with only one goal: to save my city."

"I'm not the man I was."

"You have failed this city."

"I'm a monster."

"Death doesn't scare me anymore."

"Death and I are old friends."

"Welcome to my world."

"Death is just the beginning."