<Snipped quote by Yatagarasu>
The code is needed to do the victim's job without him.
There's a lot wrong with this, then. Person one is suspicious due to not being able to give a testimony due to being unconscious. Person two knew that the control code, which is apparently related to food, was in the locket beforehand. Person three is quick to get rid of the evidence, but for some reason believes that the victim is the chemical controller.
Though if I remember correctly, the answer is that it was person three due to the victim not being dead yet.