[quote=@Yatagarasu]
<Snipped quote by Legend>

Yes, because the key didn't work. But I'm curious now on the answer.
[/quote]

The real answer is person one. They did the dishes and could spread poison on the plates. They stole person three's keys to open the chemical cabinet and didn't lock it back. The reason the key didn't work is because the cabinet was already unlocked, and person three clearly had no idea, so they have an alibi. Person two knew about the locket, but because they did laundry, they could have noticed and opened it if the victim ever left it in their pocket, so they have an alibi too. The idea that the victim killed themselves is clever, but we know it [i]has[/i] to be person one because they were faking being passed out. If they were really asleep, they wouldn't have known to take the seat they had their first week instead of the seat they had most recently.