After analyzing all the evidence, you know exactly who killed Lily, it was Brianna. She hand-made ice and mixed it with a lethal amount of arsenic to poison the drink without it being obvious, and her intention was for both of you to die. She was consumed with jealousy after seeing you and Lily get lead roles in a play with no acting experience, and it pushed her to a mental breaking point. The reason you didn't die is because you chugged the drink, giving the ice no time to melt, whereas Lily sipped on the drink for the remainder of the night. Listed below are the clues that point to Brianna being the murderer: 

• She was the only person who went to the rooms during the night, and thus was the only person that could have left the hair tie outside. 

• She made two separate trips for the ice, the first is where she grabbed the poisoned ice cubes and the second is where she got the real ice and disposed of the arsenic. 

• Brianna's testimony about Noelle going outside conflicted with Courtney's and Noelle's, as both said that Noelle immediately went back to the party.