Who really killed Melara Hetherspoon?

Question

When Cersei and Melara have their encounter with "Maggy the Frog", Maggy predicted and told Melara:

“Your death is here tonight, little one. Can you smell her breath? She is very close."

I am assuming it was really

Cersei,

though it's not really confirmed. So, what's the real story?

Who is responsible for Melara Hetherspoon's death?

Answer 1

In the case of the State (representing Melara Hetherspoon) versus Lady Cersei Lannister:

• On the charge of murder in the first degree, we find the defendant not guilty.
• On the charge of manslaughter, we find the defendant guilty.

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TL;DR: There is no confirmation on how Melara ended up in the well which she perished in, but we know for sure that Cersei made no attempt to help her, and seems to be relieved that Melara died there.

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Facts and Circumstantial Evidence

• Melara Hetherspoon fell or was pushed into a well at night time, some time after visiting Maggy The Frog's tent.
• She died in that well.
• Cersei Lannister was present at both locations.
• Melara revealed that she had feelings for Jaime Lannister during the visit to Maggy's tent.

• Cersei claims that Melara "fell" into the well; which is something she only does when she is speaking about the incident to a third party.

  To Lady Taena:

  Joffrey had never had a close friend of his own age, that she recalled. The poor boy was always alone. I had Jaime when I was a child . . . and Melara, until she fell into the well.
  ^{-A Song of Ice and Fire - Book Four: A Feast for Crows, Chapter Twenty-Eight (Cersei VI).}

  and to Qyburn:

  "This maegi made certain prophecies. I laughed at them at first, but . . . she foretold the death of one of my bedmaids. At the time she made the prophecy, the girl was one-and-ten, healthy as a little horse and safe within the Rock. Yet she soon fell down a well and drowned.
  ^{-A Song of Ice and Fire - Book Four: A Feast for Crows, Chapter Thirty-Six (Cersei VIII).}

• Cersei remembers Melara's final look as an "accusing one"

  (during her walk of Atonement)

  A young girl sat beneath a fountain, drenched in spray, and stared at her with Melara Hetherspoon's accusing eyes.
  ^{-A Song of Ice and Fire - Book Five: A Dance With Dragons, Chapter Sixty-Five (Cersei II).}

• Maggy told Melara what seems to allude to Cersei being the cause of her death:

  “Not Jaime, nor any other man,” said Maggy. “Worms will have your maidenhead. Your death is here tonight, little one. Can you smell her breath? She is very close.”
  ^{-A Song of Ice and Fire - Book Four: A Feast for Crows, Chapter Twelve (Cersei III).}

  • There has never been a time within the entire series where "death" has been referred to as a "she". Sure, The Seven are in fact One, meaning that the "Mother" and the "Crone" are synonymous with the "Stranger", but in this case it's too specific.

Means

Melara was not a large person, given the correct bump, Cersei could have feasibly pushed her into the well. Additionally, if the fall was in fact an accident, it's entirely possible that Cersei was able to either stifle or ignore any cries for help from Melara.

In fact, Melara did shout

She could still hear Melara Hetherspoon insisting that if they never spoke about the prophecies, they would not come true. She was not so silent in the well, though. She screamed and shouted.
^{-A Song of Ice and Fire - Book Four: A Feast for Crows, Chapter Thirty-Nine (Cersei IX).}

It can be assumed that Melara drowned once fallen into the well:

Cersei had not had a friend she so enjoyed since Melara Hetherspoon, and Melara had turned out to be a greedy little schemer with ideas above her station. I should not think ill of her. She's dead and drowned, and she taught me never to trust anyone but Jaime.
^{-A Song of Ice and Fire - Book Four: A Feast for Crows, Chapter Twenty-Four (Cersei V).}

So all Cersei had to do was, nothing.

Motive

It is widely established that Cersei Lannister believed that her and her twin brother, Jaime Lannister, were "meant for each other". She was very possessive over him. So once she found out that Melara had feelings for Jaime, she became distrustful and vengeful towards Melara:
(at Maggy's)

"I get three questions too," her friend insisted. And when Cersei tugged upon her arm, she wriggled free and turned back to the crone. "Will I marry Jaime?" she [Melara] blurted out.
You stupid girl, the queen thought, angry even now. Jaime does not even know you are alive. Back then her brother lived only for swords and dogs and horses... and for her, his twin.
^{-A Song of Ice and Fire - Book Four: A Feast for Crows, Chapter Twelve (Cersei III).}

Opportunity

Cersei Lannister and Melara Hetherspoon were close friends and spent a lot of time together whilst growing up. Cersei, being born into privilege was free to move in any direction she pleased, even at night.

Additionally, being that the pair were presently attending the Tourney for Viserys, and their parents being pre-occupied with the major events, they would not have been under constant supervision.