Read the Control Room Log carefully.

Read the Employee Medical Information carefully.

Read the Scent Enrichment Guide carefully.

Cleaning staff—who were in the tiger control room shortly before the killer was—noted a strong eucalyptus scent when they left the room (see the Control Room Log). The eucalyptus scent would have still been there when the killer entered the room.

Deven can be eliminated because he couldn't have entered the tiger control room (like the killer did) without having a severe allergic reaction.

Deven is allergic to a compound in eucalyptus (see the Employee Medical Information and the Scent Enrichment Guide—the compound is 1,8-cineole). Airborne exposure is enough to trigger Deven's allergy.

When cleaning staff left the tiger control room, they noted a strong eucalyptus scent that persisted even after they'd sprayed Febreze in the room. The killer entered the control room 8 minutes after the cleaning staff left, so the scent would have still been present. Deven couldn't have entered the control room, so he isn't the killer.