There's been a lot of death on Arrow, and a few have been really significant and rather tragic. I've resisted putting Sara Lance's in the list, because it gave cause for a lot of misery that ran throughout season 3. The death of Moira Queen, however, was a catalyst for Oliver being able to overcome Slade, and her sacrifice saved the life of her daughter Thea. The matriarch of the Queen family may have made some extremely poor decisions, especially for her part in the Undertaking, but here she was righting those wrongs, and showing just how much she loves her children, by choosing herself to be killed by Slade Wilson. It was a truly shocking moment, as you watched it expecting Oliver to come to the rescue, or Slade not to go through with it. It showed just what the villain was capable of and how far he'd push Oliver, which really helped set up the rest of their rivalry, and aided Thea's development as a character.