Hmmm. Poison works on weight so you would need way too much poison. that would be too expensive. Just lock it in and a lack of water would finish this off in a few days. It not going outside for a few days would not be too weird, I am doubtful if it even is possible to go outside for this creature.  Plus, who would miss this? And yes I do over analyse murder plots but I'm probably not actually a psychopath