The real culprit (spoilers)

Amy killed them. She put the rat poison in their cocaine. She had access to it because she was already involved with Dave. The giveaway is her speech at the memorial service for the twins. She comes to the stand very early, second I think, and is obviously smug and sarcastic with "too bad I didn't get to know you better." She wanted to hang out with them, they ignored her so she killed them. And it's probably her that stands behind Robert with a mobile phone in the final scene.